White T-shirt. Painted by with red, black and white fabric spray paint.
I initially tried to do a design under the 666, but I didn't like how it looked so I covered it with white paint, and then put black and red stripes overtop. It is slighly noticeable that something used to be there, as well the fabric is a bit stiffer due to all the layers of paint.
Machine washable, hang to dry.