You are partially correct.... Blenders are usually tone on tone, or have tiny prints that don't stand out. They're usually used for backgrounds or to compliment the louder, more vivid prints in a quilt, and in my opinion because they add texture, they're much better than plain solids, which to me look flat and boring.
Yes, color depends on what's going on in the project, but most blender lines are printed in a rainbow of colors. My favorite is Grunge because as I stated earlier, I'm not a fan of girly, swirly, flowery prints. Grunge is definitely not girly, but it has a lot of texture. I know that if I look specifically for Grunge prints, it's highly likely I will find a color that will compliment and enhance my project, and add texture and visual interest. There are a couple of shops near me that carry almost every color of Grunge.