I did some digging in an old thread where we had the yellow spring flowers.

03/29/07

olly
As a youngster we would hold a buttercup under our chins. If the reflection was yellow, you were healthy. If it were bright yellow, you were very healthy. It was only later in life that I associated the tone of skin with the brightness of reflection, darker seemed brighter.

tsuwm 03/31/07

in the U.S., there is a more pedestrian childhood custom..
Like the buttercup a dandelion in bloom held under the chin tells whether you like butter if there is a yellow reflection.

-joe (and a bouquet of dandelions, for mom) friday