For colored icons 
[1] Add the xpm files in the directory where you store them 

[2] For ctwm : add in ctwmrc in Icons 

Icons {
	....
    "scilab"    "xpm:scilab-color.xpm"
    "scilex"    "xpm:scilab-color.xpm"
    "ScilabGraphic*"  "xpm:scilab-color.xpm"
}

[3] For fvwm : add in fvwmrc 

Style "scilab"     Icon scilab-color.xpm
Style "scilex"     Icon scilab-color.xpm 
Style "ScilabGraphic*" Icon scilab-color.xpm ')
