I like to color the block (or quilt) in with a drawing program on my computer, so I can quickly change colors. If the pattern doesn't come with a coloring page, I make one!
As for where to start, I sometimes take inspiration from the shapes in the block/quilt. Do I see a place for a gradient? Or maybe I want to highlight a neat design feature in the piecing.
Other times, I take inspiration from my fabric. Do I need to use up something? Is there a fabric that I fell in love with and want to highlight? Maybe I have a fabric that I haven't used in a while. Or perhaps I feel like a theme/holiday quilt.
Them I get started with my graphics program! I try out combinations to see what I like and what I don't. I try to think about the pattern being made in value, which is easy to see by converting the image to grey scale and back for a quick check. It's all about vibes there! And sometimes I'll get inspired while coloring and come up with something completely different.
Finally, I pull out the fabric and do a reality check to see if my ideas really do look like I think they will. Then it's time to cut & sew a tester!