Indeed, me three...especially as WHITE cannot be produced by an ordinary CMYK printer.
In the print industry, cyan, magenta, yellow and black are used as the primary colors. When you mix all the colors, the result is gray. Consequently you will not get anything on the paper with a basic CMYK [Cyan, Magenta, Yellow and Key (black)] inkjet or laser printer. The CMYK color mixing is subtractive, meaning that it requires the base that is being colored to have all colors (i.e., White) So that it can create color variation through subtraction:
White - Cyan - Yellow = Green
White - Yellow - Magenta = Red
White - Cyan - Magenta = Blue
White is represented as 0 cyan, 0 yellow, 0 magenta, and 0 black – effectively, 0 ink for a printer that simply has those four cartridges. This works great when you have white media, as "printing no ink" simply leaves the white exposed, but as you can imagine, this doesn't work for non-white media.
If you don't have a base color to subtract from (i.e., Black), then it doesn't matter what you subtract from it, you still have the color Black.
RGB color is the mixing of light and is additive mixing so that Red + Green + Blue = White
Now there are special printers which can operate in the CMYW color space, or otherwise have a white ink or toner. These can be used to print light colors on top of dark or otherwise non-white media. These solvent and "eco" solvent printers include white and metallic in the inkset configuration, including printers from Epson, Mimaki, Roland, and many more. These printers rely on software RIPs to provide instructions to these printers. In addition to this, the files need to be "set up" properly using spot color layers or alpha channels in various image editing or drawing programs.
At this time, only Epson sells a white print cartridge for use with its inkjet printers. This option might work for small print runs on your home printer, but the cost of the white ink is much higher than typical inks.
The Epson Stylus Pro WT7900 is over $3,000 and a full set of ink cartridges is over $1,200. The white ink cartridge alone is about $300.