I'm thinking about the green and purple for between the rows of stack n whack blocks, but it seems to "heavy" to me, so I may have to use them for the borders. I may use white, or find a complementary yellow for between. I don't have any more of the yellow, bought this MANY years ago, so it might have to be something new. I don't like the original fabric between either--but it might be useful for an accent somewhere.
Here is an idea of what the finished top is supposed to look like. Check out Judy's blog to see what others are doing on their design walls.