They are blocks. They are a solid color. That color can be changed via unifieddyes's airbrush. That is all. They don't do anything else.
(and yet somehow they have proven to be rather popular on at least one server)
Solid Color Blocks
Colorable (via unifieddyes) solid-color blocks.
Reviews
Review
Awesome mod for...
...minimalistic builders like me. The only "bug" I noticed is that when you place the "base" block it blinks in pink color for a second and then turns white, I don't know why.