// Attach this script to a camera and it will paint black pixels in 3D 
    // on whatever the user clicks. Make sure the mesh you want to paint 
    // on has a mesh collider attached.
    function Update () {
        // Only when we press the mouse
        if (!Input.GetMouseButton (0))
            return;
        // Only if we hit something, do we continue
        var hit : RaycastHit;
        if (!Physics.Raycast (camera.ScreenPointToRay(Input.mousePosition), hit))
            return;
        // Just in case, also make sure the collider also has a renderer
        // material and texture. Also we should ignore primitive colliders.
        var renderer : Renderer = hit.collider.renderer;
           var meshCollider = hit.collider as MeshCollider;
        if (renderer == null || renderer.sharedMaterial == null ||
            renderer.sharedMaterial.mainTexture == null || meshCollider == null)
            return;
     
        // Now draw a pixel where we hit the object
        var tex : Texture2D = renderer.material.mainTexture;
        var pixelUV = hit.textureCoord;
        pixelUV.x *= tex.width;
        pixelUV.y *= tex.height;
        
        tex.SetPixel(pixelUV.x, pixelUV.y, Color.black);
     
        tex.Apply();
    }

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Update() {
        if (!Input.GetMouseButton(0))
            return;
        
        RaycastHit hit;
        if (!Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), out hit))
            return;
        
        Renderer renderer = hit.collider.renderer;
        MeshCollider meshCollider = hit.collider as MeshCollider;
        if (renderer == null || renderer.sharedMaterial == null || renderer.sharedMaterial.mainTexture == null || meshCollider == null)
            return;
        
        Texture2D tex = renderer.material.mainTexture;
        Vector2 pixelUV = hit.textureCoord;
        pixelUV.x *= tex.width;
        pixelUV.y *= tex.height;
        tex.SetPixel(pixelUV.x, pixelUV.y, Color.black);
        tex.Apply();
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Update():
        if not Input.GetMouseButton(0):
            return
        hit as RaycastHit
        if not Physics.Raycast(camera.ScreenPointToRay(Input.mousePosition), hit):
            return
        renderer as Renderer = hit.collider.renderer
        meshCollider as MeshCollider = hit.collider
        if (((renderer == null) or (renderer.sharedMaterial == null)) or (renderer.sharedMaterial.mainTexture == null)) or (meshCollider == null):
            return
        tex as Texture2D = renderer.material.mainTexture
        pixelUV as Vector2 = hit.textureCoord
        pixelUV.x *= tex.width
        pixelUV.y *= tex.height
        tex.SetPixel(pixelUV.x, pixelUV.y, Color.black)
        tex.Apply()