Final answer:
To add unselected pixels to a selection within a color range, use the Select > Grow command in an image editing program.
Step-by-step explanation:
If you're working in an image editing program and have made a color-based selection but notice that there are stray, unselected pixels within the selection that should have been included, the best command to use would be Select > Grow. This command checks around the already selected pixels and adds adjacent pixels that fall within a specified color range to the selection. Commands like Select > Similar could also be used if you are trying to select pixels throughout the entire image that are of a similar color, but Select > Grow is more precise for expanding an existing selection in a localized area. Select > Modify > Smooth and Select > Modify > Expand are useful for different purposes; smoothing out the edges of a selection, and expanding a selection by a specific number of pixels regardless of color, respectively.