so we've got a problem hueston?????

For trails, you need to use the ColorSequence instance. ColorSequence allows trails to change colors for certain keypoints over time. You need to put Color3 values within the instance. If you want to keep it as a solid color throughout, you do it like this.

Player.Character.Head.Trail.Color = ColorSequence.new(Color3.fromRGB(0, 150/255, 0))

If Player is already defined, then the issue here is that Color is meant to be a Color3:

Player.Character.Head.Trail.Color = Color3.fromRGB(0, 150, 0)

On the other hand, if the Player is not defined, then run this as a local script under StarterPlayer:

local Player = game.Players.LocalPlayer
repeat wait() until Player.Character ~= nil
local Trail=  Player.Character.Head:WaitForChild("Trail")
Trail.Color = Color3.fromRGB(0,150,0)