what is microline good for?
It's just a rock.
When setting up my Control Room... for all the various levers that control the functioning of my fort... I try to make each lever out of an appropriately-colored Mechanism. That way, I can tell at a glance that this RED bank of levers (made from Kaolinite Mechanisms) controls the Magma pumps, and this BLUE bank of levers (made from Microcline Mechanisms) controls the water pumps, while this BROWN bank of levers (made from Schist Mechanisms) controls my array of wooden cages containing Puppy-Bombs, etc.
Naturally, I add <N>otes as well, but the color-coding directs my attention to the proper part of the control room.
Another use for colored stone is inlaid mosaics. You can build a constructed floor directly on top of a natural floor, so you can easily turn a big open room into a picture of burning elves, or a representation of your favorite kitten sandwich, or whatever.
Workshops can also be color-coded. Masons are white, Carpenters are yellow, Craftsdwarves are blue, etc... and by using appropriate stone, you can color-code the workshops in the same pattern.