Instead of my standard inexpensive craft paints or affordable large volume custom-mixed gallons from Home Depot or Lowe's, I went with Artists Acrylics from a nearby Aaron Brothers art supply store. The main reason for this choice is it's what recommended on the "Mother of all Terrain Board building sites" -- at least in my own humble opinion -- over at Clarence Harrison's Quindia Studios.
Since first creating this blog back in the Summer of 2010 I've always featured a link to this incredibly useful site, but here's another one:
QUINDIA STUDIOS ARTICLES - including how to make Terrain Boards
(If you're interested scroll down to the long row of thumbnails second from the bottom of the page and start clicking!)
I went with Liquitex DEEP TURQUOISE blue, RAW UMBER brown, TITANIUM WHITE & MARS BLACK, and GREEN, BROWN, YELLOW OCHRE, BLACK & WHITE; and Windsor & Newton YELLOW OCHRE, BURNT UMBER red-brown, and PALE UMBER drab beige.
For the "blending" process where you're trying to naturally blend colors together to go from a darker center where the water is DEEP to lighter colors towards the riverbanks on both sides, they worked well...