In broad strokes, an OLED turns current into visible light. More current = more light, and more current = more power. So the brighter an individual pixel needs to be, the more current is consumed by the OLED cell. There is a generally linear relationship between the brightness of a display and its power consumption.
In discrete LEDs the datasheets will typically have a table or plot of luminosity (candelas) vs current through the device, and you can figure how much current is needed (and therefore how much power is drawn) to reach a target luminosity. Brightness (nits) is a measure of the luminosity over an area. The tables in the datasheet of an OLED display unit are a bit different but the principle holds.
Now going from RGB color space in the digital world to the actual current draws of individual LED cells is not a 1:1 or even linear relationship, so while you can say in broad strokes that #ffffff will draw more power than #000000 it's not as straightforward for the rest of the color gamut. The reason to keep that in mind is that the "brightness" (or saturation, if you're in HSV instead of HSL) is not equivalent to the physical brightness of the display.
