// 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.textureCoord2;
pixelUV.x *= tex.width;
pixelUV.y *= tex.height;

tex.SetPixel(pixelUV.x, pixelUV.y, Color.black);

tex.Apply();
}