I believe they take the promo chips away when you lose. So if you bet $20 on red and $20 on black, if it lands on red, you should keep your promo chips and get $20 in cashable chips. You would lose your $20 in black promo chips. You would keep your cashable chips and never bet them again.
But I have never done this before so I am not sure if they only give you promo chips if you win.