Not quite sure what you are asking, but it looks like you want to take input like
$input="blue,red,green";
And then create a drop list from that.
I'd use the "explode" function to break the string into an array
$colors=explode($input, ",");
And then loop thru the results.
For ($i=0; $i<sizeof($colors); $i++)
{
echo "<OPTION NAME=\"whatever\"". VALUE=$colors[$i].">".$colors[$i]."</OPTION>\n";
}
HOPE THIS HELPS!
-BEN