[Fou-Lu]

Colour => number map it PHP side. Ignore the 1-3.

RewriteEngine on
RewriteRule colours/([^/]+)/?$ colours.php?s=$1

PHP side:

PHP Code:

<?php

if (isset($_GET['s']))
{
    switch ($_GET['s'])
    {
        case 1:
        case 'red': // PHP can switch on strings.
            print 'provided "red"';
            break;
        case 2:
        case 'green':
            print 'provided "green"';
        break;
        //. . .
    }
}

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.

Try that.