Hey there! I have a question, if you don't mind! I saw a picture of Orion on APOD and there were so many colors behind that constellation, like red nebulae... Why can't we see it in the sky?
Hello! Good to hear from you again, & of course I don't mind!! That's what I'm here for.
Was it this image? Assuming it was, it's an image captured with the Hubble Space Telescope. One of the very interesting facts about Hubble's imaging system is that it does not capture coloured images, it captures black and white images.
Most of Hubble's coloured images are a combination of two or more black and white exposures, and then colours are added during the image processing. Usually these colours are enhanced, and aren't always necessarily what we'd see with naked human eyes if we looked at these parts of the sky from space. [Or even with the best telescopes.]
As you can see in the screenshot below, there are many different reasons why certain colours are used and/or why they are enhanced.
You can learn more about the literal science behind Hubble's coloured images here on their site.
I hope this helped clear things up a bit for you, thanks for the ask!!