It could be that way, if it was important to use color for some other purpose. I would have like to use color to indicate the category of the special, but since some specials effect lots of meters, that won't work.
"Green=Good" is certainly more intuitive that "Square=Good"
The shaped backgrounds could and was meant to be in addition to colour, not to replace it.
That said, having all good specials be green might be limiting... Various icons might be odd-looking if forced to be green instead of their more natural colours. Having the icons be various colours would also make them easier to identify.
Perhaps the background could be coloured and shaped to indicate quality of the special, but the overlaid specific image / object could be of any colour.
Square = Good might not be obviously, but what about Circle = Good, Square = Neutral, Triangle = Semi-Bad (isoceles, wide base, like traffic warning sign) .... ?
Edit: also, it'd be good if the specials worked when shown smaller than 32x32, which is a fairly large size for an icon of this sort. Rather than a planet outline with a smaller graphic inside, the distinctive graphic should dominate, so that when scaled down, it's still the most prominent part of the icon...