Minimalist IKEA Cabinet Hack, and lots of other ideas on hiding a litter box. Not just for the litter box but for a dog's or cat's pet bed, when company is over or just during the day, just shut the door and it won't have to be seen until bedtime.
Grey bedroom - could add a splash of color to bring it up-to-date! Really nice idea and love the colour scheme, but could maybe soften the grey a little so it's not so harsh. Love the white bedding and black-framed photos.