Here is a poor screen cap from a Youtube video.
Why do these icons change colour? What significance does this have?
It's my understanding that this indicates that the target is (or is currently) immune to the effect of the condition in question. However the condition is still on the target, for the purposes of applying once their immunity ends or triggering combinations with other effects or skills, et cetera.
This may happen due to the Resistance boon, for example (but may possibly also be innate to some creatures).