Most of the time when I complete a word, it is grey in the list. Every so often, it is green, red, or yellow. What do these colors mean?
If you go to:
Help > Learn to Play > Word Colors
Then it details some different colours that are used for certain words (although it does state these are used on the results screen, I would assume the app is consistent in highlighting words). This is as follows:
There is no suggestion of yellow, although when highlighting a word it will flash yellow by default if you have already found that word. And I can only assume a grey colour indicates the word does not fall into any of the other criteria.