I use Walmart polyethylene tarps on the floor. Fairly inexpensive and can be used more than once — cured resin patches/blobs/drips can be popped off.
Tint and opaque pigment are 2 different beasts. Tint is a see through/clear coloration.
I find adding a little white opaque pigment to other colors of opaque pigment increases opacity. But it will make your color lighter.
Again test panels are your friend. But getting representative proportions right is more difficult for small test batches of resin.