Sample replacing RenderTexture with InkPainter.

ReplaceRenderTextureSample.cs

using System.Linq;
using UnityEngine;
using Es.InkPainter;

public class ReplaceRenderTextureSample : MonoBehaviour
{
	[SerializeField]
	InkCanvas canvas;

	[SerializeField]
	Texture replaceTarget;

	RenderTexture texture;

	private void OnGUI()
	{
		if(GUILayout.Button("Replace render texture"))
		{
			//create render texture.
			texture = new RenderTexture(replaceTarget.width, replaceTarget.height, 0);

			//target convert to render texture.
			Graphics.Blit(replaceTarget, texture);

			//set render texture.
			var paintData = canvas.PaintDatas.FirstOrDefault();
			paintData.mainTexture = replaceTarget;
			paintData.paintMainTexture = texture;
			paintData.material.SetTexture(paintData.mainTexturePropertyID, texture);
		}
	}
}