You can get a Duotone effect using Adjust/Hue-Saturation.
If you upgrade to 6.5 you’ll get CC Toner, a three-colour tinter that works like a Tritone setup.
If you have access to Chris and Trish Meyer’s Creating Motion Graphics books (and anyone serious about using AE should own both of them) the cover CD on one of ’em includes a similar 3 colour tint plugin from Boris FX.
Trapcode Shine can work as a great 3 or 5 colour tint plugin. Just turn the Ray Length and Light Boost parameters to 0.