There is a white option, so he is free to use that. The uniforms may be modified (within reason!) so if he requires white trousers and boots, that is allowed. As long as it follows the color scheme (white, black, purple) and maintains the ALASTAIR logo, you're fine!

As for the uniforms being mandatory, they are not strictly required, but they are the only clothing articles that will replenish themselves in characters' closets. No other clothing will be provided free of charge; characters will have to hope that different worlds have clothing they can purchase, which may not always be the case. In addition, the uniforms are fairly durable (fire-, water-, and weather-proof; stain-resistant; etc) so they are recommended.