Colour => number map it PHP side. Ignore the 1-3.
RewriteRule colours/([^/]+)/?$ colours.php?s=$1
case 'red': // PHP can switch on strings.
print 'provided "red"';
print 'provided "green"';
//. . .
You can use a db or array as well. The idea is that you can match either the number or the corresponding name. If you only have RGB, I'd suggest leaving it as just a switch since that'll make it easier.
Now you should be able to use colours/blue or colours/2 and result in the same output.