soil color and soil quality

I'm wondering if there's a correlation between soil color and soil quality (i.e. quantity of beneficial microorganisms, etc.)  From my own amateur experience it seems like good soil is almost always dark brown, while barren dirt is much lighter.  Compost, of course, is nearly black.  So is it really that simple?  Does darker = better? 

Yup - that's a common observation.  Might not be 100% accurate, but when I see dark, I think "soil" and when I see light I think "dirt".

although this is a slightly different take on the question you've posed, I've read that watering the garden in the morning is a good thing, because it darkens the soil and thus heats up better, translating into a better growing environment for the plants, rather than watering at night, which can lead to powdery mildew, increased slug activity and other not very attractive things... 

Looks like there's actually some scientific evidence backing this up.  I just found this article which suggests that soil color is a reliable indicator of both organic content and carbon levels.

darker soils might mean more organic content but dark soil or soil color has no bearing on the content of minerals or lack of. i would still do a soil test to see what I have to much of or not enough of so I would know what amendments would need to be added in order to grow properly.