I obsess for hours/days, and buy what i think goes together, then take it to my Viking dealer who takes one 10 second glance at the stack of fabric and says something like "get rid of the dark purple and get a light blue that matches the flower in the print" She's always right. Sigh.

I don't use anything, I just eyeball it :lol:

I have scaned fabrics and turned the JPG into grayscale. Saves on ink.

My ace in the hole is my hubby. He's color blind (red/green if that makes a difference). I pick the colors and he picks the order from light-to-dark. After I put the blocks on my design wall I get DH to rearrange them. He sits and points while I move them around.

He's my Ruby Beholder!

Take photos of your fabrics laid out together and change it to Black and white. You will see how things pop/blend together.
Most phones have a BW option on their camera, and its an easy way to do this!

Also, after laying out blocks for placement, do the same thing, and you will spot any 'hot' spots...

When I was taught to read xrays 30 years ago, we did approixmately the same thing. You look at the film up close looking for specific things, then you stand back a few feet and take your eyes out of focus--many times this is how you spot masses or lesions.

I do the same thing with fabric. I lay it out in approximate sizes to what I am going to use or just the bolts edges arranged in what I think is right. Then I step back several feet and unfocus my eyes.

bargello quilts really need these skills..if you don't get the hues/values just right, you have a muddy quilt.
same with watercolor quilts!