heat value in each color in sunlight - science demo doesn't work well

For a high school science demonstration, I've made
up a panel of shallow boxes, each with a thermometer
and different color internal back (white, black, red,
yellow, green, blue) and put them in the window
facing the sun. The temperatures rose about 40deg C
in 15mins or so (even at 10am in winter, ie low
elevation sun, but full sun) but with only about
10deg C between the hotest and coolest box. The
green box is about the same temperature as the
black box.

I had hoped that the white box would rise little in
temperature and the black box the most with the other
colors having temperature increases according to the
number of photons at each color. While the order of
the differences are approximately what I expect
(over about a 10deg temp range), the temperature
rise in each box is swamped by a 30deg increase for
all boxes, which doesn't fit in well with what I
expected (I had hoped the white and yellow boxes
wouldn't rise much in temperature at all).

The only explanation is that all the colors are dark
in a photon sense (ie are absorbing most of the photons)
and only reflecting back a relatively small number of
photons of the color I see. The green color is relatively
dark for instance, but the yellow looks (to me) relatively
light. However maybe I don't know what bright yellow
looks like.

Does anyone have an idea on how to do this better?
ie how do I make a (Stephan-Boltzmann) white box?
It seems like all the boxes I've made are black
(or nearly so).

Box construction:

The back is a 24" x 9" *** of foam board (1/4"
thick styrofoam with paper on both sides - makes it
easier to cut and stiffer than straight styrofoam)

The 24" x 9" *** is divided into 6 side-by-side boxes
with 1/4" high walls made of the same foam board.
The back of the box has a piece of paper of each
color (or none for white). The thermometer (alcohol
in glass about 6" long, mounted on 1" wide strip
of Al metal) is mounted on top of the paper and
a plexiglass *** (24"x9") is placed on top of
the boxes, sealing them all (ie each box is air
tight). The whole thing (24" x 9" x 3/4") is hung
in the window.

Thanks Joe

--
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
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