After examining my photos, it's a Yellow-bellied Sapsucker at Denver City Park on this Sunday. (I'll post pix on eBird later today.)
Streaky bits of red coming in on the crown are mixed with the "yellow scallops" that Cornell Birds of the World describes for a YBSA juvenile; there is none of the dark crown cast attributed to Red-naped.
Pretty sure it's a male. Early red at chin and neck are muted, almost washed, but no room for the white of a female.
Also, FYI, I misremembered a date in my original post:
I saw adult Yellow-bellied and Red-naped sapsuckers a week apart in City Park in April 2017, not 2018.
Incidentally, that was the first eBird record of Red-naped in the park (with a second sighting of the same bird reported a week later).
It was just the second Yellow-bellied sighting, so today's makes the third on record.
Patrick O'Driscoll
Denver