Belkar was casting from a scroll he found searching Durkon. White is Durkon's casting colour.
Secondly, Belkar wasn't his normal self at the time - the "Owl's Wisdom" spell seemed to have temporarily given him a Celia-like value-system, as well as making him smarter.
Its an interesting question in itself - is Belkar evil only because he's stupid? Is it all just rage against a world he lacks the wisdom to understand?
Conversely, if Elan weren't brain-damaged, would the think like Nale? Did his bullying twin brother accidentally impair the "be an evil bastard" part of Elan's brain?