Why is my wall grey and how do I make it white?

I was trying to make a white wall as a backdrop for taking photos. When I left the texture blank & colour white, the wall was grey. So I put on a white texture but the wall was still grey. I thought it was maybe due to shadows (the position of the sun) so I played around with all times of the day or night. Nope, it stays grey. Then I rotated the wall and found that it has to face West -- only the side that faces West will be white.

Anybody know why this is and how I can make my wall stay white all the time? Thanks

If you want a white-white backdrop for photos, select the surface and tick the full bright box. (that's on the same tab as the textures)

Thank you SO much, Viktoria! That did the trick!! I'm so happy, you made my day