You can force a specific width or height by putting a "!" after the value of pixels. If the image can not be fit to the height and/or width dimensions you specify without changing the images aspect ratio, it will be padded out to that number.

There is a color setting in there as well but I don't have a admin page handy to look at. If you are still unsure after some time, post again and I will look. (or some one else may jump in and say.)