Can you tell me how does that work? For example, I want to make planetary nebulae more diffuse-like.
This is impossible with current version.
Another thing, where can i find the other types of nebula? I've only found the planetary ones
Increase galaxy magintude (Shift-]), then you can see pink dots - these are the diffuse nebulae, increase it some more, and you can see green dots - these are the planetary nebulae and supernovae remnants. Disable star clusters before you do this though.
Here's a pretty nebula. How about this one?
This is bad example. Firstly, this is Cassiopeia A - a supernova remnant, not planetary nebula. Secondly, this is a multi-wavelangth composite image, red channel is infrared, green is visible, blue is x-ray. Here is visible light image from Hubble:
SpaceEngine shows you the Universe only in visible light spectrum, so look carefully at textures that you want to use for creating objects. It is done for correctness, to avoid deluding other users.
Here is an example of a planetary nebula in real colors (visible light):