As the name suggests, Biangular can be viewed by holding the prism at 2 different viewing angles; horizontally and vertically. Depending on which angle you are viewing it at determines what the resulting colours will be.

Prism Held Horizontally
Typically when viewing prism art we hold the prism horizontally, and when the prism is held this way, the colours in the triangles turn blue and yellow as shown below:

Prism Rotated 90 Degrees Counter Clockwise
If you rotate the prism 90 degrees counter clockwise from the original horizontal axis so the prism is vertical, you will see the colours change to red and cyan. It should look like this: