Photoshop :: Color Select Via Wand Select?

I have two questions for CS4, which may not be possible, and I didn't find in the feature forums. 
 
1) Is there a way to do a color range selection created from all the colors that were selected from the magic wand tool.  See attached image one.
 
2) Is there a way to crop an image based on histogram?  For example if I wanted to remove all px that were above level 200 for all colors (or a specific color).  Can effect the image at all from the histogram (without an side step to something like select color range)?
 
I'm working with very large images on multiple systems (both 32 and 64 bit, all CS4).  Normally in the 50,000x30,000 -- 50,000x70,000 px range.  For the sake of argument and ease of use I could scale down but eventually need to be operating in this range.  The images are created by stitching a series of images taken under high magnification.  I would like to find ways to quantify certain regions of interested based on color, but that do not have great color separation (see image 1).  The hope would be take this information from several images and export the histogram information.  As far as cropping the histogram is concerned: the scanning process used to create the stitched images produces a lot of near white color variation that so far I've been unable to select entirely (via color range select using clusters).  I would say my best effort is only 85% of the "white" (see image 2)  I want to drop out the white px for size reduction and noise reduction when it comes to histogram statistics.
 
Also I know this is pushing the limits of intention for PS, but we tried this on three other software packages-seemingly designed for these purposes--and failed.