Yes, but it’s also a hack and therefore unsafe. Mistyping the color or icon name or having Coda evaluate it before you have a chance to finish typing it in will crash your doc.
and a few answers below there’s a list of values to use for an icon.
I personally prefer using emoji in button labels instead of setting one of those 20 button icons. Also keeping buttons white but coloring the label is a nice (and safer, and less performance-heavy) alternative