I can offer a little more on the underside of the tongue than what you've presented above. Perhaps Chad will chime in after me.
In so far as color, you are correct, the darker the veins, the more blood stasis is involved. If they are pale, grey, and distended, it's been taught to me that this involves more dampness and phlegm accumulation. If you see a grey pallor with dark purple / black veins running through, then you probably have a combination of things that are stuck (I'd guess yellow hue to the tongue tissue would indicate phlegm, too, possibly with heat, but I've never heard of yellow on the underside of the tongue).
I pay attention to the areas of distention, and correlate the root (framing the frenulum) with the Lower Jiao, the middle with the Middle Jiao and toward the tip with the Heart & head in the Upper Jiao.
Lastly is the amount of distention in the veins — the wider the vein, in theory, the more the stagnation.