Actually the wings are just acryillic paint like 
the rest of the cel.  I got the effect by placing 
dabs of yellow and white paint close to each 
other, then making two swirls so that the brush 
had both yellow, white, and a mix of the two.  
Then using fairly even stroks I filled in the 
wings.  I repeated this process on both wings and 
just used the rest of the now mixed white/yellow 
for the second and third layers.

I'm glad you liked the effect and thanks for the 
link!

Codax