Most boas will change color, some more so than others. Usually around a shed, a boa will darken up. Gravid females also tend to be darker. Some will darken and lighten throughout the day - appearing one shade during daylight and another in the evening. Some will just randomly darken and lighten. It can be a very subtle change or glaringly different. I've got one boa that can look almost anery in her dark phase, but be quite light & bright in color at other times. With her, there seems to be no rhyme or reason for the changes as they don't consistently change with shed cycles or day/night cycles. Just some days she's light, some days she's dark. The rest of my boas also lighten and darken at different times, though none of the rest to the degree she does.
Hope this answers your question. 
Raven