How do i add colors to certain ranks?

Good on ya for attempting to write the code for this. I understand not knowing where to look, and having everyone shouting at you for not knowing how to do it.

Anyway, you can get their rank's name using the GetRoleInGroup method of their Player, with the group ID given as an argument.

Using this, and a table of possible text colors, you can set their text color like so:

-- Put this script in the StarterGui.
-- Then change the GroupId to your Group's Id.

local GroupId = 1141296

local TextColorByRank = {
    "Member" = Color3.new(1, 1, 1),
    "Administrator" = Color3.new(1, 1, 1),
    "Owner" = Color3.new(1, 1, 1),
}

while not Game.Players.LocalPlayer do wait() end
local plr = game.Players.LocalPlayer
local head = plr:WaitForChild("Head")
if plr:IsInGroup(GroupId) then
    local gui = Instance.new("BillboardGui", head)
    gui.Adornee = head
    gui.Size = UDim2.new(100,0,1,0)
    gui.StudsOffset = Vector3.new(0,3,0)
    gui.Name = "RankGui"
    local rank = Instance.new("TextLabel", gui)
    rank.Name = "Rank"
    rank.BackgroundTransparency = 1
    rank.BorderSizePixel = 0
    rank.ZIndex = 2
    rank.Font = "ArialBold"
    rank.Text = plr:GetRoleInGroup(GroupId)
    rank.Size = UDim2.new(1,0,1,0)
    rank.TextColor3 = TextColorByRank[plr:GetRoleInGroup(GroupId)] --The line I changed.
    rank.TextStrokeColor3 = Color3.new(0,0,0)
    rank.TextStrokeTransparency = 0
    rank.TextScaled = true
    rank.TextWrapped = false
end

Do note that this code will break if a player in the group joins with a group name that isn't in the TextColorByRank table. You will have to add checks for that yourself.