When I practice one hand not all the keys in green or blue are correctly highlighted.
How can I change them? The zoom option change only the visible region of keyboard, but doesn't change the gray keys.
when I play a song only a part of the keyboard is clearly visible, the other keys are grayed out.
Unfortunately, often the gray keys should be visible because they are part of the hand I want to exercise and therefore should not play.
How can I solve the problem?
What are you using to play music in Synthesia? Do you have a digital piano connected? Or are you using your typing keyboard? (Or, I suppose, are you using a touchscreen?)
Hmm, strange. In "Watch and Listen Only" mode, the gray keyboard darkening should never appear. Which version number do you see on the title screen (or in the top, draggable application bar)? Do you have 10.4?
In your case -- without using any input of your own -- the only difference between the two modes is that one shows the gray keyboard shading and the other one doesn't.