The VMU, or Visual Memory Unit is a memory card peripheral for the Dreamcast. While its most basic function is as a removable storage device, the VMU may also serve as an auxiliary display during normal gameplay and, through use of additional software, act as a handheld game console. Console-like features of the VMU include a small black and white screen, speaker, directional pad, four buttons and the ability to download additional games. Initially only available in white, colors were expanded to transparent Red, Blue, Green, Yellow, Black and many other color variations.
Source: Wikipedia, "VMU", available under the CC-BY-SA License.