You can type something in a cell, then Format|Cells|Alignment to display the
text on an angle (use the Orientation part to tilt the text).
The fill colors fill the cell. But if it's really important, you might be able
to add some shapes (via the Drawing toolbar) like triangles that fit over your
cell. Then color each triangle separately.
You can turn off the lines on the triangles. And if you use the alt key when
you're positioning the shapes, they'll snap-to the cell's borders. (And set the
transparency to 50% (about??).)
