I agree with Patrick, the colour filters are a little hit and miss. Personally, unless one is really focussed on enhancing subtle features on planets (in an unnatural view from a colour perspective, which impedes the effect, in my opinion) then I’ve always felt the colour filters weren’t really for me.
I have too many filters to use, left alone justify. If i were to recommend one to start with as a general purpose filter aimed at enhancing sky and nebula contrast, it would have to be the UHC (irrespective of brand, but you really can’t go past Astronomik). If your scope has a reasonable aperture and you observe under reasonably dark skies, then an OIII filter will compliment that well. If I could only have one though, it would definitely be the UHC filter. It has a less pronounced effect when compared to the OIII, which works on more nebula in typical urban conditions under typical urban skies. The OIII can tend to block the view if the conditions and scope aren’t quite right. the Hb filter is really only effective on a select number of targets, the most popular being the Horsehead Nebula. This is an object that won’t be seen by simply applying a HB filter. Dark, transparent skies and a reasonably large aperture are the only things that will give you half a fighting chance for this particular object... the HB filter will then hopefully increase your chances of seeing it, but will add nothing if all other conditions aren’t right.
Note, I have never seen the Horsehead, irrespective of effort and owning several filters, including the HB!
