Pictured is the basic scheme I'm going for.

My question is how do I use a light color for the recessed areas where I normally drop in nuln oil? How do you make the recessed lines clean? I looked at a painting tutorial for the WH40k model but I dont think I can use the same techniques since the x-wing model is so small.

You can make a pseudo wash of a lighter color and paint in the recessed areas.

I did it with white on this Scum Headhunter (on the wings).

I followed up with a dry brush of the black. Also, it may not look too neat in the photo, but at table top distance, it looks good.

Otherwise, (just spit-balling, mind you,) prime in your lighter color if you can, then come back and paint on the raised surfaces in the darker color.

Not sure if that helps.