PS
# ShadedSector.m4
# https://tex.stackexchange.com/questions/635327/multi-color-circular-gradient-in-tikz

gen_init
NeedDpicTools

 rx = 4
 n = int(rx/lthick)
 for i=1 to n do { left_
   hsvtorgb((1-i/n)*180,1,1,r,g,b)
   arc rad i/n*rx with .c at (0,0) outlined rgbstring(r,g,b) }

PE