This thread has just explained a lot...
We keep thinking Dusty and Bump look more/less identical by the day.
Looks like Dusty just gets lighter than Bump does (the best way to tell them apart is that Dusty is always slightly lighter - more so than usual at the moment, whereas Bump is either all darker or just has light patches behind her ears).
Fortunately even when they are looking identical there're behavioural differences between them so we can always tell them apart if needed