In general terms, I view rivers as being more green and brownish than lakes, which I view as more blueish.
Assuming you're digging out a trough for the river and will be using some type of acrylic pour for the actual river part ( I like WS Realistic Water), I would get a medium to dark brown and a medium to dark green latex or acrylic paints and paint the edges of the trough with the green and the very center with the brown, making sure to blend the colors together where they meet.
1. Don't forget to seal the edges of the river on the layout with some joint compound so the "water pour" doesn't leak through.
2. Glue debris (logs, branches, fallen trees, rocks, etc,) along the river banks or in the middle, before the pour for some added realism.
3. You can also tint the "water" with a little of the green paint if you like before the pour.
4. After the pour dries, go over it with a little Modge Podge and a brush to give the surface some texture.
Lots of videos available (including WS) to help out.