Is it possible to change the colour of a moving object as it moves?

I have a video of water moving along a track. There are circles on the track, which have been located using the imfindcircles function in each frame as the video progresses. I essentially want the circles to disappear (not be found) when water has moved over them. I have tried varying the sensitivity and edge threshold of imfindcircles, however the circles must be too similar since there is no value where the ones with water over them are not detected whilst the others are. The water is slightly flourescent, so it can be seen moving in the video. Is there a way to cause the water to be a different colour/be darker once it has moved, so the circles are not found once it has moved over them? Thanks

[Andrew Palmer]

Don't worry about an answer, I found a method to do this