How do I make a pair of math.random() generated numbers not repeat itself?

First off I would like to say it is important to include the following at the top of your script:

math.randomseed(os.time());

This makes the random numbers even more random.

Then add this: (sorry for weird formatting, I am typing this in web browser and not in an editor)

So basically what you wanna do is store all the random colors in a table, and when you generate a color, you want to check and see if it is already in use.

local colors = {};

function IsInTable(Table,Item)
	for i = 1, #Table, 1 do
 		if (Table[i] == Item) then
			return true;
		end
	end
	return false;
end

function GenrateRandomColor()
	local color = Color3.new(math.random(),math.random(),math.random());
	while true do
		if (IsInTable(colors,color) then
			-- Another player already has this color
			color = Color3.new(math.random(),math.random(),math.random());
		else
			return color;
                end
    	end
end

function InsertColor()
    for i, v in pairs (game.Players:GetPlayers()) do
    local PlayerColor = Instance.new("Color3Value")
    PlayerColor.Parent = v.Character
    PlayerColor.Name = "Color"
    local RandomColor = GenrateRandomColor();
    PlayerColor.Value = Color3.new(RandomColor.r, RandomColor.g, RandomColor.b)
    TweenColorScreen()
    end
end

EDIT: I left out a wait(); at line 14 because it is unlikely you will need to iterate more than once to find a unique random color.

So recap:

- Use math.randomseed(os.time()) to ensure your numbers are actually random

- In the extremely unlikely case that a player will get the same three multi digit / decimal random numbers as someone else, keep track of all the colors and check whether or not it has been used yet, if so generate a new one and repeat.