Generated once again with my homebrew Acidity software, and a Gaussian blur & resample in GIMP.
q = 1 / (r + df)
v = (q%f)/(f*sin(f*theta+q)
v is the color intensity. df is a small value to avoid division by zero, % is fractional modulo, f is the number of spiral arms, and r and theta are the independent variables on a polar graph.
Formula is the result of a typo when copying an older formula, which I am quite sure I wrote at 1:15 AM while waiting for the dryer to finish and being almost asleep.