What is the best material to color a very large poster?

I bought this poster from five below and it's very large. Its one of those where its already got a design drawn and you just color in the lines. I'm not sure what material would look best to color it all? I've been considering using oil pastels I'm just not sure what would make the colors look most even and not cheap? Thanks in advance!

lwasmychild2016-07-07T23:28:27Z

Colored pencils probably. Maybe marker. You could also do (if you want it to look artsy) oil pastels or crayon and then watercolor overtop. That way it doesn't need to be perfectly even, and it'll still look good.