This slip up occurs in Season 2, episode titled "The Feeling That We Have".

The 1st minute in, you will see Cassie painting a beautiful portrait. Look beneath the person's jawline (in the portrait). It is empty. The camera moves back to the portrait and we see that Cassie has filled it in with Green. Her boss speaks to her and we see that once again, it's white with only a little green. When her boss walks away, it's completely white, no green at all.