White is not considered a color when printing fine art or photographs using the giclee method of photo printing processes. An area in your image which is meant to be white will actually show in the print as an absence of any inks or dyes. That is because the digital printers used for photo and fine art printing do not include any white color.
Traditionally the systems used to print, process a combination of colors such as cyan, magenta, yellow and black, combined at different levels to achieve various tones and hues. Modern printing systems now have adopted some additional colors to extend the depth of possible color but it is impossible to mix the different colors and get white.
So if you have a painting you want to reproduce but you used a very bright white paint like a titanium white in parts of it, you will not be able to get the same level of whiteness in a print unless the paper you choose has that same level of whiteness as its base.
This poses a potential problem for some artists since they want to get matching colors in their prints. To top it off, the higher grade papers for giclee printing tend not to be as bright as the lower grade papers (see my post about choosing papers for printing giclee prints).
Even though this is the case, all is not lost. Our eyes and brains are trained to take in the entire picture within context. Therefore even bright white fluffy clouds on a blue sky, on an off-white paper will be seen as bright.
There really is not much of a downside to this seemingly lack of white being printed except in cases where you are producing a print on a non-white surface. As an example, our wood prints and silver based metal prints do not allow you to achieve much in the way of white. If you were to print an American flag on it, the white strips would be either the wood tone of the wooden panel or silver of the aluminum panel. In some cases, that sort of effect is exactly what he artist is looking for however keep this in mind if you do not want that base to replace your whites.
In recent years there have been efforts to include white as an ink color. This has had mixed results in my opinion and still more suited for commercial printing where things like cards, brochures and banners are printed on a colored paper stock. Usually this entails printing a layer of white over the media, then applying the colors over that.