How a FLASH PRINT looks like
A flash print consists of two layers: a design that is normally visible and a design that is visible during flash photography. As with general printing, the "normally visible design" is less visible when it is closer to white and more visible when it is closer to black. The design visible in the flash is visible through reflected light, so it is easier to see when the image is closer to white and harder to see when it is closer to black. It's a similar idea to the way a TV monitor looks and the way light looks.
Designs that are visible in normal times are often brighter around them, and the closer the color of the design is to black, the easier it is to recognize them. When your surroundings are dark, it's harder to recognize the whole thing.
The closer the design is to white, the easier it will be to recognize the layers visible in flash photography. However, if the surrounding environment is shining strong light (such as under the sun), the reflected light will be beaten by the surrounding light and it will be difficult to see.