When first starting the Pride of the Emperor Space Marine chapter, I realized I would have to tackle something I had never experienced before- creating my own color scheme. Don't get me wrong, I've had to improvise for single models, such as my Gaulrauch the Chaos Dragon conversion into a Fateweaver, but never on the grand scale of an entire army. I had a lot of questions that needed answers before I started painting squads.
So where did I start out? First off I took a look at the overall theme of my army, being lion themed. As I mentioned previously I used to play a trading card game called Legend of the Five Rings. Since there was a Lion clan in it, I used my friendly search engine to find what colors they used for the clan. The stand out colors tended to be yellows and browns. The yellows made a lot of sense since it fit so well with the lion theme. Browns, on the other hand, would have to have the right tint to contrast enough with the yellows.
After deciding on the two main colors, I headed over to the Bolter and Chainsword's Space Marine painter. I fiddled with the image for a while and realized the yellow and brown would need a dark, contrasting color to offset the other two. It didn't take long for me to realize that a very dark grey would work best. With my basic color scheme in hand it was finally time to start painting!
With those colors out of the way it now became easy to highlight the trim, eyes, and other bits. Finally, I wrote down all of the colors I used into the notebook I'm using to update the chapter so in the future I don't forget the process and have mismatched models. In the end I was very happy with the results and became excited to start the entire squad of Marines.
So to recap, start out by deciding what basic colors you want for your color scheme. Ask yourself if they hold any significance to the chapter and if so what that significance would be. Next, use a program to 'paint' a test marine digitally as it will save you time and paint in the long run. After settling on the basic colors, paint one test model to find if you like the colors and decide if you need to make any changes. Don't get discouraged and don't be afraid to make tweaks to bring it more in line with what you had envisioned. Finally, step back and look at the model and decide if it is pleasing to your eye. If not, it is best to start over at the first step and find the colors you would be happy with.
In the end, you'll be painting a lot of guys the same colors, and if you don't like how it looks, it will be awful hard to paint more!