I'd like to have cells indicating high/med/low, where low would be
represented by a blank (white) cell, med would be represented by a cell with
patterned shading (for example, blue diagonal lines) and high would be
represented by a colored cell (for example blue).

I can do this by putting values H/M/L (or 1,2,3) or something like that in
the cells, then use conditional formatting. However, I want to see only the
pattern in the cell, and not the text superimposed on the pattern. For the
solid color or blank (white), I could format the text the same color as the
shading, but how could I make the text invisible with the diagonal pattern?

Maybe there's a better way than using conditional formatting.

Any suggestions? Thanks.