[Region3] Detect if player is Region3?

You need to utilize the workspace.FindPartsInRegion3WithWhiteList function. You can call the function with the character model in the whitelist every frame or two, that's up to you to decide. Calling the function with only a whitelist that allows ~20 parts doesn't make it too expensive to run every frame. Of course if you don't need to call it that often, it's simple to optimize. Keep track of the last update and see if enough time has passed since.

local RunService = game:GetService("RunService")
local Players = game:GetService("Players")

local localPlayer = Players.LocalPlayer
local region = Region3.new(Vector3.new(-10, -10, -10), Vector3.new(10, 10, 10))

--

local function isPlayerInRegion(player, region)
    local partsInRegion = workspace:FindPartsInRegion3WithWhiteList(region, {player.Character})
    return #partsInRegion >= 1
end

local function regionUpdate()
    if localPlayer.Character then
        local localPlayerIsInRegion = isPlayerInRegion(localPlayer, region)
        localPlayer.Character.Head.BrickColor = localPlayerIsInRegion and BrickColor.Green() or BrickColor.Red()
    end
end

--

RunService.Heartbeat:Connect(regionUpdate)  --  No reason to do it before the frame is drawn by binding to the RenderStepped event, using Heartbeat here is the best