Selection by histogram is what you get with Image->Colors->Threshold. You can
either do it using a copy of your original image in RGB(A) or, to get really
sneaky, you can decompose your image into another colorspace (HSV, for
example) and achieve the effect you want by thresholding the appropriate Hue,
or Value image, then using the selection on your original image.
You said you wanted to select the colors that were "close to" white. To do
this, just use threshold on the value channel and pick up only that part of
the histogram that is "close to" white. This will give you a bi-level image
from which you can then select the white parts, using the selection on the
original image and filling with white (or whatever other color you want).
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