I cannot decide on a colour for my walls of a very small spare bedroom

Choose you paint color based on lighting, size and accessories that are in the room.View color charts and see what will work based on you home.Asking paint questions is very personal and the responses are what other like, not what will work in your home. I will tell you to be careful when choosing some cream colors as sometimes they look to yellow, grays can play brown, blue and green, blues can play on the triple side....

A Turquoise Duck egg blue. Or soft Pink or Mint Green.............

How about a very pale gray with (depending on the undertones of your curtains) a pretty blue undertone? Add some pops of color with a little black and maybe a pretty brown (to bring out the oak).

A lighter color would make it look larger. Gray is very big right now and a light color of that would look nice with oak. Perhaps add a mirror on wall to create more depth and illusion of larger room

Here is a nice color palette https://www.pinterest.com/pin/380202393536168440/ Good luck in your choice.

You could go with a darker grey on the walls. Monochromatic color schemes give a feeling of peace and calm. Then you could add a pop of color with pillows and other accessories. Behr's Tin Foil is a lovely grey that would be a little darker than your drawer fronts and curtains, but not so dark that it would make your room feel small. If you wanted a bolder color on a feature wall, Behr's Tropics or Rainwater would still keep a peaceful feeling while adding an element of drama to the room.

Pick a color you like but stay in a light color. You really cannot go wrong. I’d stay away from Gray. You have two grays already so attempting to match them would be hard and grays are a hard color because there are so many different undertones and lighting issues. Some like to blend tans with grays as they are all neutrals but I’m not a fan Of browns and grays together. Pull out a color from your bedding.