I think they're safe (one way to confirm it would be to get comparative Agility figures for white and a silver- if the colour significantly affected resale it should be apparent) because the "interesting" car market attracts the more adventurous/extrovert/fashion conscious buyer.
Couple of examples that have stood the test of time-
White Sierra looks like a taxi, but is the most desirable? colour on a Cossie (or RS Custom Escort or Astra GTE). Or vivid yellow is only seen on JCBs (guess they don't offer an alternative colour!) but it's desirable on an Integrale).