The lilac and the chocolate (if your bun is lilac as opposed to blue) are genetically linked so you can often find the dilute (the lilac) can have more browner hairs sometimes. Likewise, with the blue and the black, but the black can sometimes go brown in the sun.
It may be diet, or condition, or fur condition (such as a moult, or coming up to it) has changed, which has done this.
My smoke pearl went rusty brown because he was old and he started to turn into a sable.
Rabbit colours are fascinating.