How can I describe a prompt for red background color in Stable Diffusion Inpaint Upload function

Question

I'm using the Stable Diffusion Inpaint Upload function to generate the background for existing PNG images. I provided the init image (a PNG image with an object placed at the center) and its mask. I used the prompt "red background", but the Stable Diffusion doesn't return a red background image. How can I achieve this?