The different parts of the Soyuz rocket have different colors:
- Dark gray
Progress rockets I saw have different "liveries", multicolored like this
and white like this:
I suppose they are painted like that (sometimes multicolored, sometimes white) for a reason.
What is/are these reason/s?
I have several hypotheses:
- Visual observation. Soyuz rockets were designed at a time, when there were no rocketcams and the flight controllers used specialized telescopes to see what happens with the rocket (in addition to telemetry). If different stages have different colors it may be easier to determine whether they have separated or not. But it does not explain, why some Progress rockets (which have the same "ancestor" as Soyuz) are white.
- Danger sign. Orange parts are colored that way because some of them fall down to ground and may contain remainders of a highly toxic propellant.
- Ease of recovery. Orange parts fall to ground and are then searched for. Therefore, these parts are colored to be as easily detectable visually as possible (similar to the black boxes of aircraft). Allegedly (not sure, if it's actually true), in the 1950es the parts of rockets, which fell on the ground were collected for at least two reasons. First, in case of failure, to analyze the cause of it. Second, to prevent people from collecting the remainders of the rockets and selling them to competing nations.
- No suitable paint. Russian industry is incapable of producing a paint, which would sustain varying and harsh conditions of a spaceflight (like heat coming from the engines at the start and cold of the space in the later stages of the flight). As far as I know, you cannot paint a device the way you like - the color must be certified for the environment, in which the device will be operated. This hypothesis is refuted by the fact that the Angara rocket is white.
Update 1 (16.11.2014 15:08 MSK): ChrisR suggested another hypothesis:
- Weight. The orange parts may be bare (unpainted) to make the rocket weigh less.
Update 2 (16.11.2014 15:45 MSK): TidalWave proposed another hypothesis:
- Ice. Prior to launch, the rocket is covered with ice and therefore appears white. I. e. rockets painted with the same color may appear as white and non-white, depending on whether the ice has melted (and/or been shaken off during launch) or not.
Weight hypothesis seems to be incorrect (see comments).