Various colors are used to make it easy to fill in the colors later on. In the anime world, many assistants are paid per sheet that they fill in. The amount paid for each sheet is not exactly going to buy you a Nissan GTR but in the words of the Production IG boss - he says stick with this position for 3 years and you will be on the path to being a superstar.
He also said that many foreigners who start out with this job tend not to last long. Stick with it and it will be worth it in the long run - does not matter whether you are Japanese or not.
How many of you want to work in the anime industry? If so then you should start off with an online profile of your artwork (Pixiv, Deviant, Flickr etc) and then tell us if you think you can trace the above image within 20 mins - that's the speed expected for a piece like this. But do remember that you should not be expecting to be able to buy that Nissan GTR on your salary in the first 3 years.