astrophotography - Are all the photos of the universe really so colorful or is it just visual effects that are added later?

Many astronomical objects are too dim to be easily seen. There is no place in the universe where M42 (between Orion's legs) would look to the naked eye like it does in photos: it is too dim, and our eyes are not good at detecting colour of dim objects.

Unlike a regular camera, which has pixel detectors for red green and blue light, most coloured astronomical images are made by using a sequence of filters in front of the camera lens. The filters only let a particular colour through. The result is several greyscale images, when several images have been made of the same object, they are mixed together, by assigning a colour to each. So "red" may represent infrared, for example, a colour that cannot be seen by humans. The assignment of image colours to filters is called a pallet, and can make a big difference to the appearance of an image.

The Hubble images of "the pillars of creation" popularised a particular pallet, and Hubble site has some details on how they create the colours in images. The hubble pallet uses "blue" for a mid range filter, yellow green (this colour is produced by oxygen ions), "green" for and orangy red filte: it picks up the light from hydrogen ions, and "red" for a intense red, fading towards the infra-red (sulphur). By using "false colour" details of the distribution of ions can be picked out better, and it makes a more attractive image.