How do I fix this brick color problem?

Your script checking every child of Workspace for BrickColor, and when it detects not "Part", error occures, and script disabling, so we need to check, is current child really a part, to prevent error. Also, we need to type BrickColor.new instead of BrickColor, otherwise it won't work. So, with the following changes, your "If" line will look like that:

if child.ClassName == "Part" and child.BrickColor == BrickColor.new("Really black")  then
    --Code
end

You have a few problems. First of all, their is no reason to do wait(0), this will not do anything for you so you would need to do wait().

I guess the reason of while true do is so you can continuously have them change? that would seem excessive, if the colors will not change again, then don't run it through an infinite loop.

Also, you're not checking the color correctly on line 5, you still need to use .new, it will NOT change the part because of this, you're using == not =. You will probably have things in your Workspace that are not Parts and will cause an error because they cannot change their color, to fix this we use IsA("ClassName"), just so you know, some things you may want to change could have a different name such as UnionOperation! Edit this for your own need. Now with the information I provided, here is a script to run once when the game is started, no infinite loop:

 local parts = Workspace:GetChildren() 
for _, child in pairs(parts) do
        if child.BrickColor == BrickColor.new("Really black")  and child:IsA("Part") then
            child.BrickColor = BrickColor.new("Really red")
        end
end

Now if you want it to do this continuously , then here's the script bellow.

 local parts = workspace:GetChildren() 
while true do
    wait() -- Very short wait
    for _, child in pairs(parts) do
            if child.BrickColor == BrickColor.new("Really black") and child:IsA("Part")  then
                child.BrickColor = BrickColor.new("Really red")
        end
    end
end