It's not so much a question of color, but rather one of value.
If you divide your scraps/stash up into at least lights/med/darks and use the them in the appropriate patches, you can successfully make any block look great.
Value is more important than color.
eta:
And if by chance they all happen to the exact same value (unlikely, but bear with me), you can break them into color categories - brights vs. darks., warm vs. cool colors, etc.
There has to be some sort of contrast, otherwise you get ....mush.
And sometimes that might be what you want.....mush. Makes a great interesting background for applique.
The problem is when one doesn't want mush, and the fabrics used in a block don't have any, or very little, contrast - value or color.
Or you want a mush background, but it's all over the place and the applique pieces don't stand out - they get swallowed by all the noise in the background.
Last edited by MTS; 01-23-2013 at 06:16 PM.