Both, probably, and the pattern you described is the reassuring one. White's shift between green, brown and purplish tones, they darken when they are cooler, and they are nocturnal so a resting frog in the daytime is often darker than the same frog at midnight. Yours is going green again when he wakes up, which is the whole answer.
The cold flat is still worth fixing. Darkening with temperature is normal; sitting dark for days because the enclosure never gets warm enough is the version that becomes stress.