Color fill between two pen lines

I'm using the pen tool to make 2 curved lines. I want to fill the space between them with color to create a river/water/wave appearance. Everything I try fills the entire background when using the paint brush, not just the area between the pen lines. How can I just fill that area with color?

I was doing the same thing yesterday and was able to only fill the correct area, but today I'm unable to and it's driving me bonkers!