In display resolution a monochrome device would be indicated by how many distinct dots it would display on the screen. For a color screen we normally refer to one dot as a pixel when it is represented on an electronic screen since it may be represented with more than one dot. For example, on a TV a color pixel is built from 3 different colored dots (Red, Green, Blue). RGB is another term used to describe the bitmapped format utilizing the first letters of the three colors.
 Color spectrum
The color spectrum defines all of the colors that we can normally see in nature. On an electronic device a three color system is used to represent the spectrum. However it is not possible to show the full spectrum using this method as the various pigments and color renditions cannot reach to the corners of the spectrum space. The following diagram shows the portion of the spectrum (called the gamut) reachable by various devices used for eBook reading.
The size of the spectrum is determined by how deep each primary color can be displayed. The range is thus set by the method used to develop these primary colors. One method is called QD-LED which can reach deep reds and blues. For LCD displays the important thing for getting a wide spectrum is how white the background lamp is. The three primary colors are achieved by using colored filters so the broader the white spectrum is the better the ability to filter out the color you want. If the color isn't available in the backlight there is no way to generate it with the filter. Unfortunately the broadest background light may also consume more power and backlight power is the dominant factor in battery life. Lately the emphasis has shifted to even wider color support such as that featured in HDR and is called Wide color Gamut support.
These days the minimum standard for RGB is called sRGB. This standard was defined by Microsoft and HP back in 1996 for monitors and printers. It has now be extended to cover all screens and Internet usage. It defines the full gamut of colors, color locations for the 3 primary colors, the balance between them. It defines the values for white, called the white point, as well. It is sometimes rejected these days as not being wide enough for modern use particularly in the blue-green space. A color profile can be defined when using something other than sRGB.
 Color Definitions
There are two different color systems in use. One is called subtractive where all of the colors at once would result in black. The second is additive where all of the colors together would result in white. Subtractive is used in printing where the color is made by reflecting a certain color while absorbing others. Additive, on the other hand, is used in displays where the color itself is transmitted or obtained by filtering transmitted light.
Subtractive: CMYK, Cyan Magenta Yellow blacK, is the 4-color color space used by most printing processes, particularly offset printing. It is also the color space used by the marking engine (that part that actually marks the paper) in common inkjet printer although their firmware and drivers are designed to translate RGB (Red Green Blue) color space images on the fly. TIF has CMYK support.
Additive: RGB, Red Green Blue, is the 3-color color space commonly used for digital capture and display. The output from digital cameras is generally converted from their raw sensor data into RGB images (TIFF or JPEG) either in-camera or using external conversion software (e.g. Lightroom, Aperture, Photoshop w/ Adobe Camera RAW plugin, ...). Computer monitors also work in the RGB colorspace. There is a standard for RGB called sRGB that specially defines the bounds of the colorspace for Internet and Computer use.
For these reasons RGB is the commonly used color space for computer images while CMYK is the commonly used color space for artwork and images intended for output on printing presses. Below is a comparison of the various RGB gamut with CMYK.
 eBook use
If the cover design or images were originally for a paper book these same images may not work in an eBook. This is result of the graphic image being originally created for use on the pbook's cover in CMYK and not specifically created for the ebook. It is generally a simple matter to use an image editor that supports both RGB and CMYK to open the CMYK image, convert it to RGB, and then resave it. Conversion can result in some colors changing slightly. A good program, like Photoshop or IrfanView, will offer the ability to select the color profile (ICM) and rendering method to help minimize the changes. See grayscale for a discussion of converting color for eBook display on E Ink devices.
Sometimes the image can seem too dark in an eBook. Increasing the chroma level (amount of color) in the image will often help. This is usually better than adjusting the brightness level.