It sounds as simple as deciding on the perfect shade, but first you have to decide what 'level' of colour you want…
* If you want strong, vibrant colour that doesn't gradually fade, or to cover grey or to go lighter than your natural base shade, it has to be permanent colour. The downside is regrowth. And if you get the shade wrong, you're probably going to have to live with it until it grows out. Even colouring on top to change the shade will give a different result than if you applied the second colour on your natural hair (think of the difference when you apply pink to pale and to dark paper – the base colour affects the result).
* If you want to alter as opposed to completely change your hair colour, it's semi-permanent colour you're after. It's meant to wash out gradually over up to about 24 washes – maybe six-to-eight weeks. But you can't lighten hair with semi-permanent – you need peroxide or ammonia to take out the natural pigment, which is a two-stage process. What you can do is take hair about two shades darker, warmer or more vibrant. And while the colour's meant to wash out, the truth is there is generally residual colour left so you may still notice a little regrowth – and it does build up, which can be a problem if you want to go permanent or change colour as there's a 'barrier' of colour already around your hair. Semi won't cover grey, but it will 'harmonise' it which means it adds a little colour to grey strands to help them blend in.
* If you want just a shot of warmth or intensity that doesn't change the colour, only the tone, of your hair, you can buy any number of wash-in, wash-out colours or colour enhancers. They last for up to six or eight washes, fading gradually. They're great for making your natural colour look richer. If you use them repeatedly they do build up a small amount of residual colour, though, which can make your hair less easy to deal with if you want a longer-lasting colour on top at some point. And they have no impact on grey.