Transparancy in PS

[N-I]

When i draw on a tablet in photoshop, the pressurelevel i put on the tablet only decides the width of the brush. In some walktrough videos, i think i see transparent brushes. I know transparancy can be adjusted on layers, but i don't want to add a layer every time i change colour. Like, if i've got a background colour, and i add some other colour on top if it, depending on pressure, i will be able to see some of the background color still.

How do i achieve something like that? And is it even recommendable?

Nevermind, i found out.