[Bug: 21.5-b21] list-colors-display

[Stephen J. Turnbull]

Andreas Röhler writes:
 > 
 > Command `list-colors-display' fails with error: 
 > 
 > "Wrong type argument: stringp, ("lightgreen")
 > 
 > Diff below fixes it for me.

And probably breaks it on TTYs, where it currently works fine.  This
requires more in-depth analysis.