Being an apartment dweller, there are some limits to what one can do to bring color onto the walls. I happen to detest white walls. I experimented with ideas and came up with a quick and easy project. It doesn't leave large detectable holes in the wall and can be done quite cheaply. Most of the supplies can be purchased at the dollar store; however, I enjoyed shopping online for just the right tablecloth background. It is accomplished by simply picking up poster board, fitting it to the wall you want to cover, then gluing the tablecloth onto the poster board and tacking it onto the wall.
I used a cheap unlined tablecloth and it has held up over two years now. I do a light spritz with cleanser and gently wipe to clean and it has held up with no problem.
I found vintage valentine cards online and painted them. If you are not artistic, then you can copy the image and using a basic photo program, turned it black and white, then worked with the contrast and brightness to make it as light as possible while still being able to see the lines, and then color it. You can print it out on white card stock to make it more durable.