Hmm odd the user erased their comment but I'm not sure if this is what they were looking for with swaping to the foreground color?

https://clips.twitch.tv/BoringVenomousDonutRuleFive-2T_SYkZ5uJEicfkh

You can also swap colors like this while using the brush tool here. And even easier swap colors by holding ALT and using the mouse wheel!