It's not obvious how to find out what face it is. The usual way is to put your cursor on some text and then use
= but that's harder to do in the minibuffer. The other way I know of is to use
list-faces-display to see all the defined faces, and then you can look for something that has the same colors as the text you want to change.
In this case the face you're looking for is
minibuffer-prompt; you can set a foreground and background color there.