Are there specifications for Crewed mission spacecrafts in terms of interior and exterior paint colours?
Yes, as in the other answer, space bound equipment is a tightly controlled project with thousands of specifications for each and every aspect.
I also encountered literal 'bibles' when it came to aeronautical visual/aesthetic aspects and the space industry is no different.
For the Soviet, and then the Russian, space program, again there were specifications for everything. But when it came to interiors, for a little while they had a female architect that created some of the aesthetic aspects inside the crewed spacecraft Soyuz, and space stations Salyut and Mir, whilst also working on their space shuttle Buran:
The Soviet Architect Who Drafted the Space Race
forgotten architect of the Soviet space programme, the woman who designed the inside of space capsules.
Above: Interior design, with color specifications for the Mir space station, 1980.
“As a human, you are always looking for orientation in space. You need a top, bottom and wall.” Balashova’s solution? “She colour-coded – green floor, blue-grey ceiling, walls in a light yellow”
Above: Soyuz interior color concept in 1963.
Mir interior design color scheme work, 1976-1980.
Color specs, interior artificial leather, 1976.
(Interestingly, she also produced watercolor paintings for each space station to 'hang' in the cabins, to make it more like a living room)
Externally, the Soviets had a little variation too, with two of their space stations:
Salyut 7 at left and Salyut 6. Reason being was that for a few months both stations were in orbit at the same time, so they needed to color-differentiate between them (During initial construction, Salyut 7 was color-coded the same as 6, being its backup as was standard during the Soviet space program).
Can these paint colours be changed as part of maintenance and renovation methods with time period?
No. As others have said, it is impractical.
If the paint color of something were to change it would be because that piece of equipment, etc, was changed or replaced.