It's not CS2 but you get the idea...
Step 1: starting with a blank screen,
Step 2: Use the Polygonal Lasso Tool to select the triangle you want to fill with color (click in the top left corner, click in the bottom right corner, click in the bottom left corner, then finish back in the top left):
Step 3: Then, fill the selection with color using the Paint Bucket Tool:
Step 4: Invert the selection (Select >> Inverse or Shift+Ctrl+I):
Step 5: Fill the new selection with your other color (using the Paint Bucket Tool):
Step 6: "Esc" to clear the selection and here's the finished result:
The selection lines were too small to show up well... hopefully the text will give you an idea of what it should look like.
--Daniel