Okay, I have done this, so here I am to report what happened.
First off, if I were to make a tutorial for this, I would call it "How to tint your Lumia 521". It appears that Nokia uses a 'solvent-resistent' polycarbonate mixture that significantly reduces the effectiveness of solvents like acetone. That said, this is both a good and a bad thing. The good part is that it makes it much more difficult to melt your 521's shell and make it polycarbonate goop, or more likely, a loose fit on your phone, which would be infuriating.
So try 1: I completely screwed it up. I wanted to make Cyan and instead I got a lesson in additive color. I mixed white and blue and got orange. That makes a lot of sense since the dyes mix according to the properties of additive color. Sadly I couldn't tell this was the case by looking at the liquid and I got my phone a nasty shade of orange. I then proceeded to herp after the derp and tried to sun dry the shell. Bad idea. The sun cracked the dye residue on the shell and it looked bad.
That said, I cleaned off as much as I could from the shell and tried again.
Try 2: This time I stuck to a single color, royal blue.
Ingredients:
1 Packet of RIT dye
1.5 cups of water
0.5 cups of acetone
a pair of tongs (in my case, chopsticks)
I won't make this into a guide unless requested, so I'm just going to post pictures of the steps below (all of these pics were taken with the same 521 I was dyeing).
I also preemptively apologize for the lack of cropping. If I get any guide requests I'll actually crop them.
The flecks and discolorations on the shell are from the royal screw up that was Try 1.
The good news is, the light tint that you do get appears to be more than just skin deep. That is, it won't scratch or rub off easily. Sadly though, it appears that soaking it for longer won't get it any darker. I bet if I had a bunch of spare 521 shells, I could empirically improve the whole process. But the least I can do is create a guide if others wish to give it a shot. It seems relatively easy to do, so unless you are really afraid of failure, it's worth a shot.