G,day vector, Welcome
With a 250mm (10") dob many nebulae will show considerable detail. If you add to the equation a filter like an OIII you are looking at photographic detail
But in monochrome. With a 250mm at a dark site you may see some red in the Orion nebula (M42) and the small nebula surrounding Eta carinae is likewise orange-red. but most nebulae only register as shades of grey or green. Some planetery nebulae will present a vivid blue colour like NGC3918 in Centaurus. Ken makes a good point about training the eye's as well. even if you find a veeeery faint galaxy you can barely see, study it and ask your self questions like, can I see any detail? Is there brightness variation across the object? what shape, any colour? etc etc.
Cheers, Mick