I think this gets more into the discussion of what exactly is input. If you consider the input here to be keystrokes then yes the text box is giving feedback, but if you consider the text box not part of the game, and the input to be the string passed from the text box to the game itself then it's fine.
Let's imagine a game where you have to type something in an arbitrary text file, then when you start the game (or press something or whatever) the game does stuff based on what text was in that file. I think this is very clearly not against the rules.
So you have an html page with a text entry, a frame, and a script that takes input from the text entry and draws one of the two colours on the frame. Is the "game" here the entire webpage or just the script? Is the text entry a part of the game, or just a part of the webpage external to the game? As long as the script doesn't affect the text entry in anyway, it's only form of output follows the rules of the jam.