|MadSci Network: Zoology|
First, Jane, I checked which scorpions you can get in the Nevada desert.
This question is very difficult to answer without the species information, and after you've learned that - it's impossible!!!
Parasitism is the most obvious solution to your scorpion's problem, but it could also be that the animal had its old skin when you first saw it, then dropped it and became tan coloured.
As it's only half an inch long, and looked swollen, its likely to be moulting. I'm also wondering if you wear glasses or could have been distracted. The small scorpion could easily have been with one of its siblings, in which case you could have come across two colour variations!
Crazy, I think I am now! You probably went through all of these ideas when it happened. As it's driving me round the bend, I'll conclude by saying:
Try the links in the MadSci Library for more information on Zoology.