Yellow-green Vireo

Imo, the cause for both is the lighting. Direct lighting on the back brings out the colors, while a shadowed view of the undertail coverts softens the colors.
The face is too demarcated the crown too dark for OCWA to me.
Andy, was the bird pumping its tail up and down?

I think it's too green on the back and not a bright enough yellow on the undertail coverts or contrasting yellow to be a Palm, such as the one I just posted here. http://www.projectnoah.org/spottings/186...

Yeah, I think it's a Palm Warbler. I've seen many Orange-crowns around here and they've all been much yellower overall.

I'd say it looks most like the Palm warbler. The only real doubt I have is it's green back; the PW is brown.

It is a Palm Warbler, nominate (Western) form. Orange-crowned Warblers are rarely so drab, and never show that much of a facial pattern or breast streaking.

Looks like you got some good suggestions. If you have a copy of the Sibley field guide, take a look at the winter plumage Orange-crowns. The facial pattern is more subtle than that of a Palm Warbler and the undertail coverts are more consistent with Orange-crowns. It's also more expected in Arcata in winter.