Will Dullcoat darken?

It has for me.  Best thing to do is test it on a piece of scrap painted the color you're talking about.

Any type of clear coat you put on is going to change the appearance.

My experience with Testor's Dullcote: YES, it does yellow after time. It's not as noticeable on many typical dull or dark railroad colors. But on colors like white and yellow, you will notice it, especially if you later need to do any touch up. I will add, I've used Testor's Dullcoat for years. On a color like box car brown, you can mix the color a little easier to do any touch up later, should you need to. White, on the other hand is a another story.

I saw this quite clearly when doing some touch up on a custom painted Lehigh Valley white "snowbird" scheme engine I had done. The same white color, applied in touch up, was noticeably brighter. 

From being an artist, there have been non-yellowing museum quality spray coverings available for years from specialized art supply stores, though they tend to be pricey when compared to regular sprays.

A couple years ago, I started noticing some commercially available clear coat sprays were also being labeled as either archival or non-yellowing. I started doing some experimental sprays over white material, so I could really see any yellowing... and there was none as compared to Dullcote.

I've been using the new Krylon clear coat sprays that are sold at Walmart and have been very pleased with them. There's a gloss, satin and flat available. They are labeled as non-yellowing. I stripped and re-did my LV "snowbird" scheme engine and am much happier with the results using the Krylon clear coat.

I've never found it to be a problem except over white, and then it is slight.

In any practical sense, no. I've never noticed it, even over white, but I've never surveyed my older "work", either. Spectroscopically "yellow"? Sure, why not? Real world problem? Nah.

Krylon Clear was mentioned above. I have used it, quite a bit at one time, but sometimes found that the "flat" wasn't as flat as Dullcote. It varied. But, more seriously, paint compatibility issues bit me a few times (never had one with Dullcote; I'm sure that they exist), and, even though it was rare, it happened, and I always wondered if it would happen this time...yes, I would small-area test, but even then I had a surprise or two.

This is not to disagree with anyone else; Krylon Clear paint is good stuff, but it isn't formulated with the modeler in mind. That it is cheaper per ounce in meaningless.

Ok, I've used dull coat on large scale models since 1987.  When then changed the formula (because of the EPA) in the late 80/early 90s - I began to have "yellowing" of the dull coat after a few years.  The more time, the yellower.  The base paint is grey and it is very noticeable.