better-tee/Assets/Code/PaletteButton.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class PaletteButton : MonoBehaviour
{
    public PencilTool pencil;

    protected Image image;

    protected void Start() 
    {
        image = GetComponent<Image>();
    }

    public void OnClick()
    {
        pencil.selectedColor = image.color;
    }
}