Surface darning.

"High happiness-to-size ratio.  People ask if I did it myself...it's possible, but it would be messier, and I love the craftsmanship, like a teeny 1.5cm commissioned piece..and every time I put it on it's a split second of joy for a little irreverent rainbow breaking through my jumper...'the little rainbow that could'."

Surface darning in degradé multicoloured thread.