Bird ID puzzle!

As you can see it is mostly a gray bird, no distinct markings except for a yellow rump that is very noticeable. This is the only shot I got before it flew off. Taken yesterday.
This was taken in my backyard: Woodland Park, Colorado, elev 8500ft, Ponderosa Pine,Aspen,Englemann Spruce with small grassy meadow.

We have Yellow-rumped Warblers, but the yellow rump, beak shape/size and white eye-ring are the only thing they have in common. The bird is the size of a YRWarbler as well. Could this be an immature, or just a unique color phase?

Is this consistent with the Fall coloring of the yellow-rumped warbler? I'm not familiar with that species, but I ran across a web site showing one in Fall colors, and it seems like a possibility.

A YR Warbler would have a patch of yellow on the side and does not have a complete eye-ring. However, I can't offer a guess as to what it may be.

I'll toss out Ruby-crowned Kinglet as a possible ID. For some reason that was my first impression, but my skills with warbler and kinglet ID aren't the greatest.

Virginia's Warbler. Gray warbler with thin sharp bill, strong eye ring and yellow on the under and upper tail coverts. Yellow on the breast not obvious on a female or first fall bird.

I was going to suggest Virginia's after looking through my book but was a little unsure of the range.

Thank you everyone!

Brian, I believe you are correct, after looking at drawings of VW this does look like an immature VW.
Thanks again.

This makes 40 species seen in my backyard over the past few years since I installed a double waterfall/pond setup.

How about a immature gray flycatcher looks like it without the wing bars,and they are in that area I think,just a guess.