It’s in the documentation here.
coerceColor() is not in the documentation, doesn’t show up with code hinting, and when I run it a similar error message appears.
rs.CreateColor() was added to the Rhino 6 code stream. The Color.FromArgb() is from Ironpythons support of .NEt and the rs.coercecolor() is from the old RHino 5 Rhinoscript syntax.
All 3 of these methods result in a System.Drawing.Color.
The rs.CreateColor() will be the standard call in the future that is documented in RhinoScriptSyntax. The other two will continue to work/