While it is always best to remove a tire from the rim to blast and repaint, I find that many rims are in good enough shape to just clean up and brighten with a new coat of paint. If the tires are good, heres how I do the masking and painting without removing the tires.
Get a a roll of duck tape. Get the good stuff , not a cheepie with half the stickum on it.. The start with your index finger and push the leading corner up under the rim with the sticky to the tire. Push the sticky down against the tire, but just the width of your finger of tape. Then just continue around the wheel with two fingers holding the tape down and the index poking the leading edge down under and against the rim.
When you get around to where you started and get done it will look like this....
I usually have the top of the spray can down just in the taped circle and spray short bursts as I go around. Usually put the wheel up on a old garbage can turned upside down with cardboard under the wheel. This so I can easily walk around it as I paint. Always keep moving as you spray as runs are easy to create painting this close to the part. Get done with one side, peel the tape off, turn over and tape up the other side. If you do get overspray on the tire by chance, just have a rag handy thats wetted with paint thinner and wipe it off before it dries.
You can also stand the tire up and paint with the can vertical, just turning the wheel as you go.
If you use the lay the tire down and paint method, you can also use a piece of carboard in your off hand outside the tap to help shield the tire from overspray. At least till you get the hang of it.
I usually give the wheels 3-4 light coats, then let dry for a few days before reinstalling on the tractor