Black lambs grow into black sheep. The farmer needs a black sheep to keep the genetic diversity. I've never seen sheep at the hairdressers going for a blue rinse.
Just like al employers, farmers have ethnic quotas to fill too. The only quota they're exempt from is the disability quota, which goes some way to explain why you don't often see lambs in wheelchairs.
Quite a good joke, but I feel the need to point out that in the UK we do not have quotas in employment. Employers are expected to ensure that their recruitment policies do not discriminate in such a way as to prevent people from minority ethnic communities a) getting a job in the first place and b) progressing within the organisation. One way of assessing this is by comparing the percentage of people from specific minority ethnic groups within the relevant community with the number employed within the organisation. So its an indicator of access and discrimination not a quota system. Here endeth the lesson and sorry for going off subject. Still think it was a good joke though.