Thank you all for comments.
Jacek,
I achieve various colours using two retarders. One of them is the quarter-wave plate and another is marked IR. I don't know what "IR" means, however it isn't "Infra Red" I think. I found this plate with the old set for CZJ microscope, together with already mentioned quarter-wave plate. Rotating these two plates and the polarizer gives me wide spectrum of colours. That is all.
Marek