If you mean red, instead of real, it goes beautifully with denim and black! In fact, black and red is my favorite color combo! If you do paint the walls red, I would get bedding in mostly black or denim blue with some red accents. That would be gorgeous!

I love this bedskirt!

The red will look great with black and gray. You could paint the walls in a lighter gray.

I would definitely not get bedding that is mostly black given the black carpet because I think the room would be too dark. I would suggest bedding that is lighter in color and would re-paint the walls a light gray/greige.

I would limit the red. You could do a neutral look like Elle posted and add a few touches of red.

If you would like to add red, I would still suggest keeping the background color of the bedding light.

On my screen the curtains look more like dark gray than bluish gray. The design really depends on what they look like in person.

The pictured drapes are too short.

I made two images on the SW website. First I painted the left side of the window in Gossamer Veil. I left the right side as it is.

Then I painted one side Grand Canal and the other side in Gossamer Veil. Just so you could see a teal in proximity. I suggest neutral walls and then color for your bedding and art. It seems like teal could work. It depends on the actual color of the curtains. They do not look denim on my screen.

First find bedding you like, then choose wall color.

Some inspiration

My choice would for bedding would be a light cream with interesting texture but no other colors on it, like this from Anthropologie, so you can lighten the room. Can you lengthen your drapes a bit with a hem so they are closer to the floor? Or, make them longer by getting a nice complementary cream color to add as an accent stripe and raise your curtain rod quite a bit? I've attached some examples of stripes of fabric added at the bottom, top, and in the middle that are obviously the wrong colors but give you an idea of different ways of styling.

Last pic of fabric on curtain to extend it. Sorry it didn't attach to my prior comment.

You should go with the red or marsala. These two colors will go perfect together with these gray curtains and also give a more sophisticated and elegant look to the room.