There are brick panel kits you can purchase for inside the fireplace.

If it is a gas only fireplace, you may not be able to do any modifications without causing potential safety issues. If it is a masonry fireplace converted to gas, that's a different story. Please post some pictures of the fireplace along and see if you can find the label that details the make and model so we can offer proper advice.

You can spray paint the inside of a gas fireplace - but have a professional fireplace company do it to make sure they use proper paint and procedure. My gas fireplace is beige inside, and I am probably going to have the fireplace company spray paint it black to modernize it when I finally get around to updating the fireplace surround.

The issue is getting a lighter color. I am not aware of any high temperate spray paint that comes in lighter colors. They are only available in dark colors like black or some primary colors like engine block red.