how to tell if your fabrics have enough contrast

edited to add: Now these are all sewn together! Luckily, after I sewed the first two blocks together, I noticed that some blocks are turned the wrong way. See how the dark (green)squares in the front all march to the upper left? And the ones in the back rows march off to the right? Yeah, that's wrong.
Now it's all sewn correctly, and green goes up to the left, while all the orange marches up to the right.

(SO, SO GLAD I only  had to unpick one single short seam.)
Now I have to get the borders on - a plain one, a pieced one, and then a plain one. (The pieces for the middle border are all done.)