I think this would be difficult to achieve in the certificate module alone. Several ideas for a workaround I have in mind though:
- Make a coloured certificate for everyone and give access to the colour printer only to the users that need a coloured certificate. All others must print their coloured certificate on a b/w-printer.
- Make two different certificates, a coloured one and a bw-one. Then use groups or groupings to limit access to the certificates to the matching groups only. This might work only in moodle 2.0 or later, because of the changes of concept in activity locking and access.