Although we like to represent a bitmap image as a 2D matrix of color values, that's not actually how a display physically works. This zoomed view of an LCD display clearly shows the RGB subpixel structure of each "addressable" pixel on an LCD display. Notice how different parts of a pixel emit only red, green, or blue (corresponding to the LCD blocking different wavelengths of light emitted from the white backlight.)
You might be interested in how this physical property of a display can be exploited to produce sharper images.
Note that as high-pixel density "Retina" displays become commonplace, the value of these subpixel rendering effects diminishes since the pixels are already so small. For example, I just tried to take a look at whether my retina Macbook Pro is still doing subpixel smoothing of text. I screenshotted a region of the screen containing black text on a white background (cmd-shift-4) then zoomed into the resulting image in an image viewer. In my case, I saw no color fringing around the text so it looks like the OS is not rendering subpixel smoothing. (Note: I did make sure "Use LCD Font Smoothing When Available" was checked in the "General" tab of system preferences. I'd be curious if anyone with a non-Retina Mac can observe subpixel font rendering effects.)