The Select by Color tool is capable of selecting areas of the current layer using color. Unlike the Fuzzy Selection Tool , the Select by Color tool will make selections across the entire layer, not just adjoining areas.
There are many options that make Select by Color very powerful.
Select by Color Settings
To the left of the dialog is an area which previews the current selection using black or white. Black areas are unselected and white areas are selected regions.
There are four selection modes:
Replace: This mode will create a new selection region with each click in the image window.
Add: This mode will add to the current selection with each additional click in the image window.
Subtract: This mode removes the newly selected area from the existing selection area if it exists.
Intersect: This mode uses logical intersection to determine the resulting selection area. After clicking in the image window to indicate the new region, the selection area will be reduced to include only those regions with were overlapping. That is, where the old selection area and the new selection area intersected.
The Fuzziness Threshold slider adjusts the selection threshold. Higher thresholds result in greater areas being selected with each click.
The three buttons in this section modify the selection area quickly.
Invert: This button inverts the current selection.
All: This button selects the entire layer.
None: This button removes the current selection.
The Reset button resets the selection options to the defaults.
The Close button closes the Select by Color dialog maintaining the selection area.