Thanks for the suggestions.
I was already afraid that there are no cheap solutions. Although I would like to buy a 5D mark III. I'm afraid, my wife won't let me.
I always shoot with auto white balance. I'll try to find out how to manually determine and select the right white balance.
I like the results achieved with Topaz DeNoise. I will look into that.
I've read that using flash is usually not allowed because it might distract the players. That's why I don't use my flash.
I guess it depends on the level of the event. At this tournament, I saw someone from the judo-club that organized the tournament use a flash. I could have used mine as well.