This is something I do to help me with stuff like this.
I was painting a brick wall last year, so I went to
http://www.cgtextures.com and also googled for images of brick walls that I liked, then I went into my photo editor and pulled out the colours of various areas, and drew some squares and filled them in with that colour. Then I mixed my paints to match the colours and painted in the same places as the photos (roughly, and to taste).

OK, so it\'s a very bad photo (colour is really off in the shots of this mini!) and my painting\'s not great, but the idea works and it really helped me to not worry about the colours.