Color string can now be a char expression

Parser/Gmsh.y

@@ -5544,10 +5544,13 @@ ColorExpr :
       if(flag) yymsg(0, "Unknown color '%s'", $2);
     }
 */
-  | tSTRING
+  | String__Index
     {
       int flag;
-      $$ = GetColorForString(-1, $1, &flag);
+      if(gmsh_yystringsymbols.count($1))
+        $$ = GetColorForString(-1, gmsh_yystringsymbols[$1].c_str(), &flag);
+      else
+        $$ = GetColorForString(-1, $1, &flag);
       if(flag) yymsg(0, "Unknown color '%s'", $1);
       Free($1);
     }

doc/texinfo/gmsh.texi

@@ -1200,7 +1200,7 @@ Colors expressions are hybrids between fixed-length braced
 
 @example
 @var{color-expression}:
-  @var{string} |
+  @var{char-expression} |
   @{ @var{expression}, @var{expression}, @var{expression} @} |
   @{ @var{expression}, @var{expression}, @var{expression}, @var{expression} @} |
   @var{color-option}