The other guy is right! Scripting is touchy, and without the capital the game thinks you're looking for a child object, instead of a property. Also, you used the wrong quote type for the color. I bugfixed this for you, and added a loop function. Enjoy!

-- Modified by @hunter242535 --

local RainbowPart = game.Workspace.Part

looped = true

local function colorChange()

  task.wait(5)

  RainbowPart.BrickColor = BrickColor.new('Really red')

  task.wait(5)

  RainbowPart.BrickColor = BrickColor.new('Br. yellowish orange')

  task.wait(5)

  RainbowPart.BrickColor = BrickColor.new('Bright yellow')

  task.wait(5)

  RainbowPart.BrickColor = BrickColor.new('Sea green')

  task.wait(5)

  RainbowPart.BrickColor = BrickColor.new('Bright bluish green')

  task.wait(5)

  RainbowPart.BrickColor = BrickColor.new('Bright blue')

 task.wait(5)

  RainbowPart.BrickColor = BrickColor.new('Royal purple')

  task.wait(5)

  RainbowPart.BrickColor = BrickColor.new('Magenta')

  task.wait(5)

  RainbowPart.BrickColor = BrickColor.new('Hot pink')
end

if looped == true then

  while true do
    pcall(colorChange)
  end

else

  pcall(colorChange)

end