SyncVar Hook

The hook attribute can be used to specify a function to be called when the SyncVar changes value on the client. This ensures that all clients receive the proper variables from other clients.

The Hook method must have a single parameter of the same type as the SyncVar property. This parameter should have a unique name, e.g. newValue.
Do not try to set the property value from inside the hook. The property value will be updated after the hook completes.
Reference the hook parameter inside the hook to use the new value. Referencing the property value will be the old value, in case you need to compare.

Below is a simple example of assigning a random color to each player when they're spawned on the server. All clients will see all players in the correct colors, even if they join later.

using UnityEngine;
using UnityEngine.Networking;

public class PlayerController : NetworkBehaviour
{
    public override void OnStartServer()
    {
        base.OnStartServer();
        playerColor = Random.ColorHSV(0f, 1f, 1f, 1f, 0.5f, 1f);
    }

    [SyncVar(hook = nameof(SetColor))]
    public Color playerColor = Color.black;

    // Unity makes a clone of the material when
    // GetComponent<Renderer>().material is used.
    // Cache it here and Destroy it in OnDestroy
    // to prevent a memory leak.
    Material materialClone;

    void SetColor(Color color)
    {
        if (materialClone == null)
            materialClone = GetComponent<Renderer>().material;

        materialClone.color = color;
    }

    private void OnDestroy()
    {
        Destroy(materialClone);
    }
}

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SyncVar Hook

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