Tommy: "​ This trick is really my encore but if you like it I'll do the rest of the act. "

"Here's a black handkerchief dyed white. And here's a white handkerchief dyed black. I place them in the tin like so."

Runs hand over tin and says these magic words

"Fulla-full-full-hollo-full-to-ho. And here's a black handkerchief that's now white. 
And here's a white handkerchief that's now black. Thank-you. I'll do it again. But I'll do it a different way this time. I'll do it .........."

Turns tin slightly 

"Watch the spots."

All the spots vanish

"Aye?"

Pulls out the handkerchiefs

"Now here's the black handkerchief with white spots. And here's the white handkerchief with black spots. Howzat!!"