The text color in the Hopper and Brewing Stand GUI changes slightly based on whether or not there is an item in the player's hand.
(This also affects most inventory GUIs added by mods.)
Steps to reproduce:
1: Start a new world in Creative Mode
2: Give yourself a Hopper and place it down
3: Make sure that the Hopper item is currently selected in the hotbar and open the Hopper's GUI
4: Pick up and place down the Hopper in the hotbar slots
5: Look for text color changes
6: Repeat above steps with the Brewing Stand if you want.
(I was able to fix this in my modding environment for a GUI I'm making by disabling GL_LIGHTING before drawing any text. (I was drawing bright red text and so the bug was way more noticeable))