How do I make a brick change to a random color except a specific one?

This should work:

local brick = script.Parent

brick.BrickColor = BrickColor.Random()
if brick.BrickColor == BrickColor.Gray or BrickColor.Black or BrickColor.Brown() then
wait()
else
brick.BrickColor = BrickColor.Random()
end

This basically checks if the picked color isn't gray or black or brown. If it is, then it changes it to something new. If this doesn't please you, you can do the same using math.random to pick a random color, except the ones you don't want. So it'll be like this:

    -- this is just an example

local number = math.random(1,3)
local brick = script.Parent

if number == 1 then
brick.BrickColor = BrickColor.Red() -- so if the number 1 got chosen by math.random, it'll be red.
elseif number == 2 then
brick.BrickColor = BrickColor.Yellow()
end