Comments on the colour to use...:

taking into account the following:
- N9 has a pentile display
- generally, LEDs have different current consumption/required voltage levels per colour; red needing the lowest and blue the highest, green in the middle

It's best to use green for highest resolution / sharpest text. Note that the regular clock in white (grey) will show jagged edges on vertical lines (check at..n:11 or so).
Green is the one colour that really has the 854x480 resolution.

It's likely lowest power to use red, not sure how using blue vs green will play out - there are 2x as many green pixels as there are red or blue ones.
I wouldn't think that blue takes so much more energy per pixel that it compensates against 2 green pixels, but then the total light output pixel for red and blue pixels may be tweaked/increased to compensate for the lower amount of those..?
If so, I'd guess that blue text takes the most, then green and lastly red the least energy.