In earlier builds we had a transparent overly (colorrect) that would change opacity/alpha each wave. However it made it difficult to see the enemies well in the first few waves so at the last minute we had to cut the feature since we couldn't get it balanced in time. Since the feedback we were getting in testing was that the game seemed more challenging than we intended we decided it would be best to not cloud the issue (literally).
And as you noted, that had the consequence of making it ambiguous when the waves progressed. :) sorry for the confusion! Thanks for playing and for the feedback!