I have a for i v statement to create a GUI into PlayerGUI and I have noticed with testing on two computers that Roblox only issues the for iv statement until it is finished for each player. For example if I wanted to make a GUI appear followed by wait(3) and then get removed, it WON'T happen to all players at once. Instead, itll make the GUI appear for one player, wait(3), remove GUI, and then restart the process for the next player.....this means that if I have 12 people in my game, 36 seconds will have to go by before the rest of the code in the script executes. Why is it doing this?

for i, v in pairs(game.Players:GetChildren()) do
		local player = v:FindFirstChild("PlayerGui")
		local screengui4 = Instance.new('ScreenGui', player) 
		screengui4.Name = "playercheckgui4"
		-- Create textLabel
		local textLabel4 = Instance.new('TextLabel', player.playercheckgui4)
		textLabel4.Position = UDim2.new(0.45, 0, 0.4, 0)
		textLabel4.Size = UDim2.new(0, 250, 0, 100)
		textLabel4.FontSize = "Size28"
		textLabel4.BackgroundTransparency = "1"
		textLabel4.TextStrokeTransparency = "0"	
	for i, f in pairs(game.Players:GetPlayers()) do
		if f.TeamColor == BrickColor.new("Bright red") then
		textLabel4.Text = (f.Name .. " is the chosen one!")
	end
end

for i=1, 50 do 
	textLabel4.TextColor3 = Color3.new(math.random(), math.random(), math.random())
	wait(0.1)
end
	wait(2)
end

Code cleanup

Tab your code correctly. This lets you actually see which statements are run together in conditions and loops.

for i, v in pairs(game.Players:GetChildren()) do
	local player = v:FindFirstChild("PlayerGui")
	local screengui4 = Instance.new('ScreenGui', player) 
	screengui4.Name = "playercheckgui4"
	-- Create textLabel
	local textLabel4 = Instance.new('TextLabel', player.playercheckgui4)
	textLabel4.Position = UDim2.new(0.45, 0, 0.4, 0)
	textLabel4.Size = UDim2.new(0, 250, 0, 100)
	textLabel4.FontSize = "Size28"
	textLabel4.BackgroundTransparency = "1"
	textLabel4.TextStrokeTransparency = "0" 
	for i, f in pairs(game.Players:GetPlayers()) do
		if f.TeamColor == BrickColor.new("Bright red") then
			textLabel4.Text = (f.Name .. " is the chosen one!")
		end
	end
	for i=1, 50 do 
		textLabel4.TextColor3 = Color3.new(math.random(), math.random(), math.random())
		wait(0.1)
	end
	wait(2)
end

Use good variable names. You don't use i so use _ to indicate you don't use it. v doesn't mean anything, use player. Don't put unnecessary parenthesis around strings. Numbers aren't strings, don't put them in quotes. Use :GetPlayers() not :GetChildren() to get the sequence of players. Use one style of quotes consistently.

Don't use player.playercheckgui4. Just use the variable you have, screengui4.

After this cleanup, your code is much more readable while still doing the same things:

for _, player in pairs(game.Players:GetPlayers()) do
	local gui = player:FindFirstChild("PlayerGui")
	local screen = Instance.new("ScreenGui", gui) 
	screen.Name = "playercheckgui4"
	-- Create textLabel
	local textLabel4 = Instance.new("TextLabel", screen)
	textLabel4.Position = UDim2.new(0.45, 0, 0.4, 0)
	textLabel4.Size = UDim2.new(0, 250, 0, 100)
	textLabel4.FontSize = "Size28"
	textLabel4.BackgroundTransparency = "1"
	textLabel4.TextStrokeTransparency = "0" 
	for _, f in pairs(game.Players:GetPlayers()) do
		if f.TeamColor == BrickColor.new("Bright red") then
			textLabel4.Text = f.Name .. " is the chosen one!"
		end
	end
	for i = 1, 50 do 
		textLabel4.TextColor3 = Color3.new(math.random(), math.random(), math.random())
		wait(0.1)
	end
	wait(2)
end

Problem

Inside the main loop, you have waits. That means you process one player, wait a bit, then process the next player.

If you don't want to wait after processing each player, then that code doesn't belong in the for loop. That's why we tab our code, so that you can see what belongs where, easily.

It's not really the best that you have a for loop inside another for loop -- that's a code-smell that indicates something's not quite right. The f that you find should be the same for each one; you should just compute a chosenPlayer variable ahead of this loop.

It might look something like this:

local chosenPlayer

for _, player in pairs(game.Players:GetPlayers()) do
	if player.TeamColor == BrickColor.new("Bright red") then
		chosenPlayer = player
	end
end

assert(chosenPlayer)

for _, player in pairs(game.Players:GetPlayers()) do
	local gui = player:FindFirstChild("PlayerGui")
	local screen = Instance.new("ScreenGui", gui) 
	screen.Name = "playercheckgui4"
	-- Create textLabel
	local textLabel4 = Instance.new("TextLabel", screen)
	textLabel4.Position = UDim2.new(0.45, 0, 0.4, 0)
	textLabel4.Size = UDim2.new(0, 250, 0, 100)
	textLabel4.FontSize = "Size28"
	textLabel4.BackgroundTransparency = "1"
	textLabel4.TextStrokeTransparency = "0" 
	textLabel14.Text = f.Name .. " is the chosen one!"
end

for i = 1, 50 do 
	for _, player in pairs(game.Players:GetPlayers()) do
		local label = player.PlayerGui.playercheckgui4.textLabel14
		label.TextColor3 = Color3.new(math.random(), math.random(), math.random())
	end
	wait(0.1)
end

wait(2)

The right way

You should not be manipulating each player's GUIs from a Script object.

You should be using FilteringEnabled, and all GUI manipulation should be done by a LocalScript.

Use RemoteEvents / RemoteFunctions to communicate state to the LocalScripts.