Using Color and Grayscale Palettes

Designers often rely on color to make an application aesthetically pleasing. However, using color randomly or excessively can affect usability. To use color effectively, keep the following guidelines in mind when designing your interface:

• Display no more than four colors on a single screen at one time and limit the colors for your entire application to fewer than eight. The more color you use, the more confusing the screen will appear to the user.
• Use color in combination with other emphasis techniques to discriminate areas on the interface and identify crucial features. Never use color alone to distinguish elements, because users may have difficulty distinguishing colors in inadequate lighting. Use fonts, icons, screen placement, or patterns in addition to color to distinguish screen elements.
• Avoid spectrally extreme color combinations, such as red/blue and yellow/purple, because they can make images seem out-of-focus.
• Design applications for a grayscale display whenever possible because many users may not have color displays. Then, when the application is finished, you can add color.
• Use brighter colors for extended viewing, because dim colors may not be legible once a user's eyes adapt to the color.
• Avoid colors lacking contrast as well as colors of equal brightness, because they are not easily distinguished.
• Use black, gray, and white to improve resolution in fine detailing.
• Use common color associations, such as red for stop, or green for go, to avoid confusion.

The color design model for Windows CE mirrors the 16-color Windows palette, similar to the Windows 95 color scheme, and is measured in bits per pixel (bpp). Windows CE supports pixel formats of 1, 2, 4, 8, 16, 24, and 32 bits per pixel (bbp). Your application should determine the color format supported by a display device, and then adopt a complimentary display strategy.

Note An 8-bpp display driver can display a 32-bpp device independent bitmap (DIB) by mapping each color in the DIB color table to a specific color on the device. The palette available in the application displaying the bitmap determines what mapping is used. The application can lose color information if it does not use an appropriate palette or if a bitmap uses more colors than the palette can hold.

Standard Windows CE 16-color palette

Some Windows CE-based devices support only a 2-bpp palette, with four gray-scale colors: black, white, light gray, and dark gray. On a grayscale display, a single-pixel graphical element, such as a dot or a line, can be difficult to distinguish without a strong, contrasting color adjacent to it. For example, white and light gray elements can be hard to see unless presented against a black or dark gray background.

Likewise, light colors may be difficult to distinguish. When using light colors, you may need to double the thickness of pixels or lines to strengthen them. Light gray works well for creating a shadow effect around large controls on a white background and for anti-aliasing, which adds colored pixels to a graphic to smooth jagged edges. If you use light gray as a background color for your screen, use a white line to visually separate key areas such a command bar or owner-drawn menu from other areas of the screen.

Windows CE does not arbitrate between the palettes of the background and foreground applications. Because of this, you should generally use only the first ten and last ten colors included in the stock palette of a display device, which are generally the standard Windows/VGA colors.