pga said:
>   then final appearance is full of flickers!
>   Is there any other idea ?

move the edit field outside of the window,
by simply re-specifying it with the new coordinates:
     edit field -x, (18020,18020)-(18500,18300)

then select the desired portion, and change its color.

then move the edit field back to its position:
     edit field -x,(20,20)-(500,300)

-bowerbird