As far as paint the car a different colr, I will say what I always say and is a recurring theme here.... do what you want with the car... it is YOUR car, YOU paid the money for it so YOU can paint it any color that you like. As fas as the value of the car after the re-paint.... well, it goes both ways. You loose a little value of the car because it is not original anymore, but in the same respect, you gain value of the car because you are painting it a more desireable color. Alot more people want to buy a Black Trans Am than a red one. But it all comes down to a personall prefference. But again, it is YOUR car, do what you will with it and don't let anyone else tell you otherwise.
As far as the black paint, yes, there are different "shades" or black. The black used in 1977 was called "Starlight" black and was pint code of 19. I can get the actuall paint code if you need it.
Also, let me know when you need the decals, I can get them at a discounted price from Phoenix Graphix.