A quick little feature wall I did in my son's room using leftover paint, a sponge and about an hour of my time. It worked out well – I just adjusted how much paint I put on the sponge to get the different shades.
I found that pouring small amounts of paint into a tray worked well. Keeping the paint in the tray shallow helped with not getting too much paint on the sides of the sponge when dipping it in. When there was too much paint on the sides, it became hard to get a grip on the sponge.