Eardrum 73,
With a 300mm (12") Dob you should be able to spot many nebulae and with your size 'scope many of them will be very bright. Most nebulae appear green of grey thru a telescope. But the Orion neb (M42) may show traces of crimson red with a 300mm Dob at a dark site.
Nebula filters do indeed improve the visability of Nebulae but different viewers seem to have different experiences with them. These filters however don't bring out the colour in neb's they just allow the wavelengths of light that nebulae emit the most to be transmitted and block extaneous (ie non nebula) light. I personally have 2 filters. Lumicon OIII (oxygen 3) and a broad band filter (Its a lumicon filter but I can't remeber its name
). I really rate the OIII filter as it results in improvement on
most emission and planetary nebulae.
The links section of this web sight will give you the details of retailers that will be able to help you with filters if you wish to purchase one but first, Find that first nebula, you wont be dissapointed.
chill