See, you don't have to make it a separate step though. It'd be something that is initialized with a sane default, and allows you to change it if you want to. A button with the colour the light will be that you can click to open a modal dialog displaying your options. Someone suffering from dicromacy may choose a colour that is more distinct to them.
If you don't care about the color the light is, you'd simply confirm without changing anything.
Another nice tweak would be coloring the app based on your chosen color whilst you are waiting for the ride, making the connection obvious. That should be a fairly simple matter of shifting hues.
Like in Android contacts, when you add a new one it chooses a colour (maybe randomly, I'm not sure if there's a system for it) but you can change it if you want.
I mean it's a general interaction design principle. Provide sane defaults, offer customization. As a corollary, if you need to prompt for data, think long and hard about whether you actually need that data.
Hrm, thinking about it, it'd probably also be neat if it tracked what colours users who do choose pick, and then use that information to deduce their favorites. If any of those are available, it should pick them automatically. Even if I went out of my way to pick orange every time, it'd be nice if the app remembered that and picked orange for me, thus eliminating a step.
It may be worth displaying available colors as smaller squares next to the button like this:
If you don't care about the color the light is, you'd simply confirm without changing anything.
Another nice tweak would be coloring the app based on your chosen color whilst you are waiting for the ride, making the connection obvious. That should be a fairly simple matter of shifting hues.