During the development of the new nodes in the geometry-nodes branch
the color of the new nodes wasn't considered, so all of the nodes ended
up red, the color for "input" nodes. This patch introduces two new
colors, one for "Geometry" and one for "Attributes". There are only two
attribute nodes currently, but the next sprint will add two more,
attribute mix, and sample from texture. The attribute nodes are
conceptually different enough from the nodes that modify the geometry
that they deserve their own color