Of course you can use brushes to paint but beyond them there are a lot of other fun and different ways to make a neat painting. These are all cheap ideas you can dispose of afterward. Makes clean up a lot easier.
Put one in each color to prevent too much mixing and brown paint. It really bugs my 4 year old when my 2 year old dips his red-covered paint brush into the yellow paint, then the blue, the green, the purple, the orange, making 6 different cups of brown, brown, brown.
Cut them into different shapes and use them as stamps in paint.
3. Cotton Balls
Dip into different colors and press onto paper to make dots.
4. Toilet paper rolls
Either dip the ends in paint and stamp to make circles or dip the sides in and roll it onto paper to make blocks.
We used to cut potatoes in half and then an adult would cut a star shape out of the cut face so it was like a stamp. Then we'd stamp stars allover brown paper to make our own wrapping paper.