Rainbow smile 

to smile out of happiness while crying of sadness

she had lost all hope, but love in friendship made her dawn a rainbow smile

Rainbow smile by audiodecoy April 4, 2005

Rainbow Smile 

A euphemism for throwing up.

Dude, this kid in my class got sick and totally rainbow smiled alllll over the room. He couldn't even make it to the bathroom!

Rainbow Smile by El Chin Chon February 6, 2009