Around 65 per cent of Kiwi women dye their hair. And why not? Sure, it may just be 100,000-odd strands of dead cells, but your hair has a huge impact on how you look, how you feel - even how other people perceive you. (You know the cliches: fiery redhead, sensual brunette, gentlemen prefer blondes...) And it doesn't have to cost you much - if you do it at home. But how exactly does the dyeing process work, why does it smell so rank, and how can you make sure you go ash blonde instead of Fozzie-Bear orange? Read on for the answers...

Hair has three main layers: the outer is the cuticle, the middle's the cortex (where hair pigmentation, melanin, is stored) and at the core is the medulla. There's dark and light pigment, and your colour is determined by the ratio in which they blend. "If you have a black or dark-brown mane, you have lots of the darker melanin called eumelanin; if you have blonde, ginger or red hair, you have more [lighter] phaeomelanin," says Tony Pearce, clinical director of National Trichology Services. It took 'til early this year for scientists to discover why hair loses melanin - our follicles produce hydrogen peroxide and, as we age, the peroxide builds up, removing melanin. Which brings us to dyeing...

Chemical reaction

Let's compare the outer layer of your hair shaft to a Vienetta: layer upon layer of tightly stacked protein tiles. "To change your hair's colour, you need to lift these upper layers to let the colour molecules sit inside the hair shaft," says Bronwyn Fraser, national education manager for L'Oréal Australia. To do this, you need hardcore chemicals - ones that make your eyes water. (That strong hair-dye smell comes from the reaction with the sulphur in your hair.) The most common chemicals in hair dyes are hydrogen peroxide and ammonia. "Hydrogen peroxide is used to break up colour inside your hair shaft," says Fraser. The higher the concentration, the more pigment is removed. Ammonia causes the hair shaft to soften and opens up the cuticle, allowing the colour in.

When these chemicals make strands expand, they put them through some trauma. You may notice your hair seems thicker post-dye job - that's because your hair cuticles have lifted, like a pine cone left in the sun. "If your hair's in good condition, it's possible to rectify the damage with treatments that seal down the cuticle," says Wayne Lewis, colour director at Sydney's Atlantis Hair. But if you colour your hair all the time, cuticles can become resistant to product and you'll get frizz - your hair's way of telling you to stop colouring or go for the chop.

Think inside the box

Ask yourself how big a commitment to change you're willing to make. Then, pick a formula based on your comfort level.

Semi-permanent dye is like a summer fling - it vanishes after about 10 shampoos. Since semis don't contain peroxide or ammonia, they can't lighten your hair and the colour doesn't interact with your natural pigments, so there's no drastic transformation. A semi deposits pigment, enhancing your current shade; the small colour molecules leave the hair shaft with washing.

Want something a bit more serious? Go with demi-permanent dye. These contain low levels of ammonia, helping colour stay in your hair longer, and fade over about 25 washes. A demi can take you a shade lighter or two shades darker. Pre-colour molecules penetrate the cuticle and cortex, where they create medium-sized colour molecules. The molecules' larger size means they take longer to wash out.

Ready for a long-term colour change? You want a permanent dye. This alters your shade with peroxide and ammonia, so colour lasts until it grows out. "This product lightens the hair's natural pigment to form a new base, then adds a new permanent colour," says Fraser. This is the only choice to cover all greys.

Find the right hue

Pay attention to those tiny tufts of hair in hair-dye aisles - they're your key to nailing your new colour. First, determine your current colour using these 10 base shades, explains Anita Jelley, product manager for L'Oréal Paris New Zealand. All colours are labelled from 1 to 10, with shade 1 being black and shade 10 being the lightest blonde.

For the most natural effect, stay within two to three shades of your natural colour. "When in doubt, start with the same level or slightly lighter," says Jennifer Morgan, colourist and owner of Morgan & Morgan Urban Retreat in Auckland's Takapuna. "If the shade isn't right, it's easier to go darker than lighter."

Also consider undertones. Like your skin, your hair has them - either warm or cool - and the peroxide in permanent hair colour will expose them. "Brunettes tend to have warm undertones, which is why they're often surprised by how red their hair turns after colouring - especially when going lighter," says Belinda Jeffreys, Clairol Nice 'n Easy colour expert. If you're worried about your hair looking brassy, choose a cooler, ashier tone.

"If you have existing colour in your hair, a simple rule to remember is that colour will not lighten colours!" says Morgan. "If you've already coloured your hair before (even if it was a few months ago), stick to the same level as the mid-lengths and ends for an even result." If you want to go lighter and your hair is already coloured, leave it to a pro.

Ready...

You wouldn't slap paint on a cracked wall, right? So don't apply colour without conditioning. "If your hair is damaged, the pigment won't adhere well to your strands and it will end up streaky," says Judi Seeley, technical manager at PPS Hairwear Australia.

"At least one week before colouring, pamper your hair with a deep-conditioning treatment," says Iain Smith from ME Salon in Auckland. This will create an even surface for the colour to attach to and help protect your hair from the dye's chemicals. And don't shampoo for a day or two before you colour. "Your hair's natural oils will protect your scalp and prevent irritation," says Smith. It's fine to leave styling products in your hair though.

Get set...

Colouring your hair is like baking a soufflé: if you don't pay attention to every step, you'll end up with a mess on your hands.

"If you are going the DIY route, be realistic about your level of expertise," says Morgan. "You'll usually get the best results with demi-permanent (non-lightening) colour, which will add loads of shine without giving you a drastic change." Here, what to do before opening the box:

1. Ensure you have enough dye: "If you run short and the colour is sparse, the result will be patchy and uneven," explains Jelley. So, long-haired girls, grab two packs.
   "Apply a thin layer of Vaseline along your hairline - from earlobe to earlobe and along your neckline - to prevent the dye from staining your skin," advises Morgan. "It also pays to have some old towels in the bathroom, so you don't end up with colour on the floor or walls!"
   Mist the ends of your hair with water. "Since the tips tend to be dry and damaged, they can soak up too much colour," says Keah Fitzgerald, Australia/NZ learning manager for Sebastian Professional. "Extra moisture will help colour go on more evenly and prevent the ends from getting darker than the roots."

Colour!

Divide your hair into quarters and apply colour one section at a time to make sure you don't miss a spot. Work the colour through thoroughly with gloved hands, Jelley adds. "When you don't wear gloves, your nails will absorb the colour in a similar way to the hair - so unless you want to have your favourite shade on your nails, wear your gloves." Colour areas with greys first: they need the extra time.

If you're going darker, start from the top, front and ends of your hair, as these tend to be the lightest. If you're going lighter, start with the darker hair at the base of your scalp. If you're redoing the same colour: "Recolour the roots first and then cover the ends for just the last 10 minutes, as the peroxide in home dye is damaging on mid-lengths," says Fitzgerald. As soon as you've applied colour to the last strand, start a timer. Overdeveloping the dye could lead to dry, frizzy locks.

After dyeing, don't shampoo for two days. "This will give the cuticles - which open during the colouring process - time to close and seal in the colour molecules," says Jelley. And watch the water temperature when you wash: hot water can cause cuticles to expand and open, allowing some of the colour to escape.