The Fuzzy Select Tool is as good as the Select by Color Tool in this case.
Note also that you can change the amount of overlay to affect wider or narrower colour range.
The three prime colours are on three poles of the 360-degree circle. Blue and red will be about 120 degrees apart.
The HTML notation will be useful if you are setting colours for a webpage.
These are the RGB values.
The Autocrop feature is useful to reduce storage size by removing unused background area.