Media Encoder changing colours from After Effects

Hi there,

I'm working on a series of videos to be used in a browser and whenever I export as an .mp4 or .webm from AME it adds a strange gradient to the buildings in the video. (Please ignore the arrow and background, it's transparent)

.MOV file (correct colours on buildings):

.webM or .mp4 file (strange gradient from bottom to top on buildings):

Any ideas?

Thanks!

Sam