Photoshop Halo

Scrapping later.

I always use SAI. but recently I've been wanting to get back into using photoshop. but one thing I cant figure out is how to get rid of that darn white halo you get when trying to fill within a linework. or even filling in a block of color.

when I watched this one artist when he does a live stream, somehow he can fill in the color quickly without having to take his time coloring in with the paint brush. tried asking him..but he's just a busy guy, and dont want to be one of those people that get annoying after asking 10+ times. I even tried to do some research to find the settings or trick to it, came up empty.

How is it done! I know I cant ride off into the sunset with SAI forever. -facedesk-