Brewster's Warbler in El Valle

Last sunday at El Valle we saw an odd-looking Blue-winged Warbler. At first we thought it was a Blue-winged, but it didn't look right. It was bright yellow below, with a white vent and lower belly. Like a blue-winged, it had a dark eye-line and no black on the throat, and like
a Brewster's, it had grayish ear-coverts and was yellow on the forecrown and olive on the nape and neck. All its back was blue-gray and, like a golden-winged, had two broad yellow wingbars. Overall it looked like a Brewster's, only it had much more yellow below. Maybe it was a hybrid "Brewster's" x Blue-winged? I assume it was in breeding plumage, as its colors were very bright.
We saw it in shrubby second-growth next to the road to La Mesa, at least five times for a total observation time of about two minutes. It behaved like a typical Vermivora, gleaning insects from the bark and leaves, and foraged just above eye-level.