I am attempting to paint a Tau army at the moment for my brother. We are having a hard time figuring out what color to do these fellas in. I say paint them what ever color floats your boat.
I must agree with the folks that say you need to paint up to the lines. In our process of coming up with a color scheme we have come up with this.
And that was color scheme number 4.

I just painted the engine there over and over and over till we were happy.
The lines were KILLING me. My brother went and purchased some high quality fine tip markers and and the results are outstanding.
I say you are off to a good start and try to get you some of those pens and your life with your Tau army will be much easier.
And if you read the post I made about a Necron army, it is true. I am painting Tau for my brother and Necrons for my son and Orcs for myself. I am such a tool.

But I have found that painting these things is about 85% of the fun for me.