I have a spread sheet containing columns Budget, Expense, Variance and
% Variance in cols A, B, C and D. Col D contains formulas of the form
=C10/A10.

What code would I add to color negative percentage cell text red?

One way:
Format Cells Number Custom

and under "Type" put
0.00%;[Red]0.00%

Vary to suit.

(I have Excel 2003.)