Question:

I need idea how i can make brush who can color smudge.

Example in picture: right side painting with basic brush with two different colors in left also painting but additional use smudge tool, the result should be something like left side

I need advice how i can try to do it

Answer:

Here is one attempt

1. On mousedown grab a copy of the area under the mouse into a separate canvas

2. On mousemove draw that copy one pixel at a time from the previous mouse position to the current mouse position at 50% alpha, grabbing a new copy after each move.

In pseudo code

on mouse down
   grab copy of canvas at mouse position
   prevMousePos = currentMousePos

on mouse move
  for (pos = prevMousePos to currentMousePos step 1 pixel) 
    draw copy at pos with 50% alpha
    grab new copy of canvas at pos
  prevMousePos = currentMousePos

The brush is feathered by drawing a rgba(0,0,0,0) to rgba(0,0,0,1) radial gradient over it using globalCompositeOperation = 'destination-out'.