ausa

Hey there! I have to calibrate CCD images (ngc281) for an astronomy practicum at my university. I have three narrow band filters (H alpha, OIII and SI) and bias, darks and flats for each filter. I stacked each filter individually and put them in to the RGB and then edited the result (I'm working with SIRIL). I finally came to a more or less satisfying result, but when you zoom into the nebula the colour of the stars are somehow strange. Is that normal like this? If not, how can I change that? Thank you so much!