Here is the target (upper left), Color Mode Ia (upper right)
Color Mode II(lower left), and Color Mode IIIa (lower right)
(I had to underexpose slightly to prevent clipping across all hues so the target looks brighter than the images.)

I loaded these images into PhotoShop CS and noted the Hue, Saturation, and Brightness of each patch.
(I used the Blur Average Filter on each patch before measuring.)

In the following analysis Color Mode II is shown but we'll concentrate on the differences between Color Mode Ia and Color Mode IIIa.

Here is the plot of Hue versus Saturation.

And here is the plot of Hue versus Brightness.

Hue
On the Hue versus Saturation plot it is clear that the red Color Mode Ia squares are to the right of the Green Color Mode IIIa squares in the 240 to 300 degree range of Hue.
This fits the hypothesis that Color Mode Ia shifts magenta hues toward red.
(More pleasant flesh tones.)

Saturation
Also on the Hue versus Saturation plot it is clear that the green Color Mode IIIa squares are above the Red Color Mode Ia squares in the 60 to 120 degree range of Hue.
This fits the hypothesis that Color Mode IIIa boosts the saturation of green/yellow.
(Greener grass and vegetation.)

Brightness
On the Hue versus Brightness plot it is clear that the green Color Mode IIIa squares are well above the Red Color Mode Ia squares in the 210 to 300 degree range of hue.
This fits the hypothesis that Color Mode IIIa boosts the brightness of blue/magenta.
(Bluer skies.)

Bill

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For the most part, then, this confirms the conventional wisdom around the impact of modes Ia and IIIa: Mode I for people, Mode III for nature.

Actually I'm a little surprised by the Mode II results. The brightness values of Mode II seems to track a bit below Mode IIIa, but the saturation difference is much more pronounced than I would have expected . . . substantially more green and light (but not dark) blue. This was not clear to me from my (subjective) analysis of some crayon pictures.

tim