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I just got this guitar and I need to fix this nick on the back of the neck.

I need some advice on how I can fix this without refinishing the entire neck.

I have some regular clear nitro and smidge of tinted nitro from my Mustang project, though I am sure the original finish is not nitro.

Could I use the clear nitro to build up the area and then use tinted nitro to color match?

Without wanting to nitpick, that looks more like a chip than a ding. Dents caused by compression can often be fixed using a soldering iron and a wet cloth to expand the compressed wood. If the wood's missing then I suppose it's a different matter and you'll need to replace that material.

The neck is fine, just the finish chipped and dented. I did some light sanding to avoid more chipping of the finish and make it more comfortable.

Probably a polyurethane finish. I would tape off, with blue painters tape, around the chip. Tape to 1/8 " of chip. Then start spraying little poly coats,to the spot to build it up. The let it dry a couple days and take off the tape and wet sand the spot down to level starting with 800 grit. The poly urethane should give you close to the same amber. But you could do a test first. Sometimes at Lowes they have the finish over different types of wood to gauge what brand would be closest.

You could try the stewmac Trick

refinish the spot.

I'd do like BacchusPaul suggested and steam the dent out first. Be very careful not to heat the chipped finish around the dent.
You don't want it to start turning loose and chipping more. A damp cloth over the dent and touch the soldering iron to the cloth for a few seconds at a time.
Steaming out dents I can do. As far as finishes go. I got nothing.

do like cur said, taping the area off... but as you get almost done, pull the tape off and start working a slightly larger area, so the repair blends in (feathering).

I fixed similar laqueur chips just by dripping in tinted furniture varnish, rub flat with super fine paper and rebuff. You could see a slight line around the full, but it just looked like ageing