Why is this script not changing the colors of the text in a GUI?

Color3's work on a scale of numbers between 0-1. Since most people like to use a scale of 0-255, we need to divide every number by 255 to make it fit into the Color3's requirements.

So, let's make ourselves a function that will return a Color3 with each number divided by 255.

function ChangeColor3(r,g,b)
	return Color3.new(r/255,g/255,b/255)
end

Now, we can apply the function like so:

script.Parent.TextColor3 = Change(255,58,65) --Since the ChangeColor3 function returns a Color3, the TextColor3 will be the returned value from the function.

Finally, let's put it all together! Also, you don't need to wait while changing each property, just do it instantly and it'll work fine.

local score = game.ServerScriptService.MainGame:WaitForChild("score1")
local score2 = game.ServerScriptService.MainGame:WaitForChild("score2")

function ChangeColor3(r,g,b)
	return Color3.new(r/255,g/255,b/255)
end

while wait() do
    if score2.Value > score1.Value then
        script.Parent.TextColor3 = ChangeColor3(72,255,46)
        script.Parent.Text = "Winning"
    elseif score2.Value < score1.Value then
        script.Parent.TextColor3 = ChangeColor3(255,58,65)
        script.Parent.Text = "Losing"
    elseif score2.Value == score1.Value then
        script.Parent.TextColor3 = ChangeColor3(124,156,255)
        script.Parent.Text = "Tied"
    end
end

We can improve the script by making the code a function and then connecting it to the Changed function of each score value.

Final code:

local score = game.ServerScriptService.MainGame:WaitForChild("score1")
local score2 = game.ServerScriptService.MainGame:WaitForChild("score2")

function ChangeColor3(r,g,b)
	return Color3.new(r/255,g/255,b/255)
end

function updateScore() --updateScore is a variable we can use to call the function.
    if score2.Value > score1.Value then
        script.Parent.TextColor3 = ChangeColor3(72,255,46)
        script.Parent.Text = "Winning"
    elseif score2.Value < score1.Value then
        script.Parent.TextColor3 = ChangeColor3(255,58,65)
        script.Parent.Text = "Losing"
    elseif score2.Value == score1.Value then
        script.Parent.TextColor3 = ChangeColor3(124,156,255)
        script.Parent.Text = "Tied"
    end
end

score.Changed:connect(updateScore) --Connects the score's Changed event to the updateScore function
score2.Changed:connect(updateScore)

I hope my answer helped you. If it did, be sure to accept it.