Oh yeah, having spent almost a month on this figure going in to mess him up is terrifying. That's one of the reasons I start small and work up. A little mud on the boots? No big deal. After that it gets a bit tougher. At least with the dry pigments you can wipe a lot of them off if you're not happy. Of course, if you were to use a darker pigment on white, like your astronauts, Demi, even wiping it off would still leave a lot of color behind. In that case I'd start with light brown and grey, then build up to darker colors depending on how things looked.
Practice on space marines or fantasy infantry or whatever non-display stuff you've got on the table. Once you sort of know how it will turn out then you can try it on the display figures. Still, even with some experience I still start my weathering by saying "dear god, don't let me screw this up!"
Dennis, I'll take a closer look at the elbow. I kind of liked the stronger weathering there but I'll give it some more thought.