FireClient: player argument must be a Player object?

Assuming you are using a Server Script , This is what you should be doing.

game.Players.PlayerAdded:Connect(function(plr)
local RS = game:GetService("ReplicatedStorage")

local eventz = RS.RemoteEvents:WaitForChild("ForAdmin")
-- You cannot use LocalPlayer in a server script
eventz:FireClient(plr)
end)

Also may you please elaborate on argument #2 (852,0,.207,0)

FilteringEnabled

With FilteringEnabled being enabled, there are some mandatory restrictions. One of these applies to the script you're using, a Server Script. The change is that you cannot access LocalPlayer from the server. It will always return nil, which is why you are receieving. The player parameter you pass must be an Instance, instead it is nothing. nil. You will need another method of obtaining the player, such as the PlayerAdded event.