That's a great idea!!!!!
I've got to give that a try today!
A great use for all the scrape pieces of tube I have laying around.
Do you get a good color reading? Thinking outside the box I'd say take it one small step further and lightly scuff an area on the back side of the viewing area. This should give you a nice opaque white background to see the color against. You could tape off (with masking tape) a rectangle area so that you had clean lines for the scuffed area. I wonder if white paint would stick as that could be another way.
Now I just have to figure out how to try this out on Valentines Day.