Color fill between two pen lines

[Maeta5E55]

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! 

[c.pfaffenbichler]

If you want to paint you need a Vector Mask in which the two pathItems are linked up to form one shape. 

Edit: A Layer Mask would be another option, but that would not make it easier in my opinion. 

[c.pfaffenbichler]