I noticed that usually movement lines are marked yellow, however sometimes they switch to blue. Spindizzies seem to switch back and forth for each hex, but what's the difference?

yellow is the normal movement your engines provide while blue is the added benifit of dark light. For every hex of blue it cost 1 darklight to move in gav 1, 2 in grav 2, etc. . . Spindizzies alternate to show the different system being used.

that explains so many things.

on the subject of colors, I've gotten that the different colors of grav well are different "densities," and that increased gravity increases difficulty to move... but what's the code?

each one increases as you go starting with the light red. if you hit show coord and hover the mouse over the hex in question it will tell you the gravwell strength and coordinate of that hex. they typically increase by one per band as you approach a planet.

I'd figured the increasing bands- the problem is the overlaping zones where dark red shows up (it's both 1 and 7, so I wasn't sure if they were adding or negating)...
thanks for the tip on using coord to find the gravity value- I'd used it everywhere except a gravwell, so I'd not discovered that.