c# mixing of multi panel selected colors

Question

Dear Friends,

 

Please help me the code of below given details

i have 4 panels and selected colors using colordialog control.

if i used differnet colors then output is like given image.

if i selected same colors then output is same colors in drwlines.there is no effet .only look solid effect. 

 

finally when i click picturebox then output is like draw lines with multi color effect.please ref image. 

 

please share me the code?