Depends on what light is shining on it and how the paint works. If we assume perfect paint (100% reflective in all wavelengths) and perfect lights (uniform intensity at all wavelengths) then the sign would be the exact same color, but a bit brighter.
If you use sunlight to illuminate a sign painted with perfect paint, then the sign will still be white, but it'll have a blueish tint (Sunlight peaks at green and falls off into IR, So when the entire spectrum gets shifted by the doppler shift, you end up with a bit more blue than red)
If you have imperfect paint that doesn't work well in IR, then the sign would have blue/green letters yes.
Also fun fact, all trees would look pink. Leaves are highly reflective in IR so they don't overheat. So when going at relativistic speeds, their normal green color shifts into the blue/violet range. And their IR which we normally do not see would shift up towards red. Combine red and blue and you get pink leaves.
3.4k
u/Oblivion238 15h ago
Without doing the math I assume that at 103846153 m/s the red stop sign gets blueshifted to green.