Why do some brushes have orange backgrounds and I can't paint with them

Hi!  I am a newbie here and to Corel Painter.  I bought Corel Painter 2021 at the end of last year and it is only now that I am starting to work through a course on the application. I have noticed today that e.g. under Liquid Ink Brushes that 4 and a half of the brushes have orange backgrounds and I can't paint with them. I am sorry, I know that this is probably a stupid question to probably all of you but should that be the case and what can I do to rectify the problem please?

Thank you in advance

Sr. Sabina