'Idol' Chick -- Amazing 'Grace'

3/18/2010 2:03 AM PDT BY TMZ STAFF
Here's "American Idol" standout Siobhan Magnus on Tuesday night's show (left) -- and Debra Messing as Grace Adler in a flashback episode of "Will & Grace" (right).

Siobhan Magnus and Debra Messing
One of them brought out the Lambert cackle to sing "Paint it Black" in an '80s prom dress on Tuesday.

We're just sayin'.