How to color you knee?

We went for a ride on Saturday. The weather was ok for riding. But when we got to the forest the trail was muddy and slippery. The bike was all over the place o the downhills and on the climbs the tires were struggling for some traction. On a slippery descent the rear wheel slipped and i hit my knee with the bike. It was so painful... After a few km the pain went away but after a few hours my left knee was swollen and it was colorful (mainly red and blue). A few pics from the road:

It's good to see other people in the forest :)