I like the idea of always on top. I'd put the checkbox there, so the people could set it to the place they'd want it. Also try to fix this: when you fill the fields of the RGB and forget to click on the RGB button (so U have the GAMMA active), this will automatically change you the RGB values you wrote a while ago, so I'd like sth like a autoswitching RGB-GAMMA button there which would make the field in which you are working active.
Also if you fill the RGB fields and there's an empty gammafield (there is not a zero but totally anything) and press the convert button, it will call you an alert window. Ex: RGB: 122 154 168 GAMMA: nothing 3220 -458 will alert you. It would be better not to do this and convert the values even if there's anything in one of the fields. Good luck!