Here's my pair of P mearnsi. Notice the contrasting
coloration/pattern in both the male (right) and female
(left).

Also their tails are not as spiny as the Guyana collared
swift (AKA "spiny tailed collared".

Most collareds (Crotaphytus) have much smoother
and smaller scales throughout ... especially in their
longer tails.

All good guessed though.
I am strongly leaning toward the Guyana collared swift
on this lil female.

You decide,
John