I want to set a datum elevation and have any contour below that red and any contour above that green.
Currently we are getting something like this pic...where about half the area should be green and half red.
How about creating 2 surfaces, defining them by including pts above (or below) your datum elevation, and editing the properties of each to make one red and one green?
you would probably have to delete some lines of the tin triangles to ensure that the surfaces don't overlap.
It's lots of work for something simple and there is probably a better way but this should do the trick.
In surface properties you can try these settings. Click on the "Color Scheme" color bars to change the color. Change the max elevation for 1 and the min for 2 to be your datum elevation. I am using 2013 but I believe it is the same in 2012.
Scott
Try Nick DellaRosa Cad support at Gateway. The guru for Civil 3D