i dont know!!!!!!!!!!!!!!!

I am not sure if I am correct but you get from a treasure chest.
Go South near the stadium where you find Bowser Jr. Go through the warp pipe. You will see
a treasure chest. Defeat the Yellow Shy Guy and get Wario to open it and there you have
it. Give it back to Red Yoshi and you will unlock him. He also suggest that you should
return it to the true owner Green Paratroopa in Wario City. Once you do that he will ask
you to paint a new billboard for him. Once finished you've unlocked him aswell!

yea thats where u get it

to get brush

Go to yoshi park and after you enter go south and you will see one of yoshi's manholes and
below it is yoshi's ballpark. use yoshi to go through it then where bowser jr was (near
the pipe) defeat him then use mario to go through the pipe then use wario to open the
treasure chest. (the paint brush is in there)after return to red yoshi and tell him you
have found it