Getting together a "theme" is as important a part of the project as any other.

Choosing colour is usually straightforward. 

If I'm knitting for someone else, I'll usually ask them to show me something the colour they want, or I'll bring them bits of wool I already have and ask how close that is to what they want. If there's more than one colour, it's important to see how they'll look together.

Colour choice can also affect the price - different companies have different ranges of colour in different types of yarn. There are usually a few options, it's just a case of finding the balance between the shade you want and the price you like!