Histogram giving too many pixels

Question

I am using photoshop to design rugs and i have to get an accurate percentage for each colour used in the design so that the rug makers can dye  the correct amounts of wool.
I am simply converting the image to indexed colour and making sure that if it is a three colour rug there are only three indexed colours in the design.
then i am selecting colours using the magic wand and working working out from the histogram what percentage of the image this colour is by dividing the number of pixels selected ,by the noncontiguous non anti aliased magic wand, by the total pixels in the image.
most of the time this is working fine, but sometimes one area of colour will clearly be read as having far too many pixels, sometimes even more pixels than are read in the image as a whole.
I don't understand why this is happening, there are no layers in the image, surely this should be really straightforward?
please help!

Answer 1

If you want to give a Script a try, paste the following text into a new file in ExtendScript Toolkit (part of Photoshop’s installation, Applications/Utilities/Adobe Utilities/ExtendScript Toolkit CS4 or /Applications/Utilities/Adobe Utilities-CS5/ExtendScript Toolkit CS5) and save it as a jsx-file into Photoshop’s Presets/Scripts-folder.
After restarting Photoshop the Script should be available under File > Scripts and can be assigned a Keyboard Shortcut directly, recorded into an Action, (in CS4 and CS5) be used in a Configurator-Panel or started from ExtendScript Toolkit directly.
Edit: Quickmask might be a better way to avoid crowding the History, so I’m changing the code.
// alert of percentage of selected pixels relative to whole image, but only if selection is strictly black/white;
// 2011, use it at your own risk;
#target photoshop
if (app.documents.length > 0) {
var myDocument = app.activeDocument;
// from function hasSelection(doc) by xbytor;
var res = false;
var as = myDocument.activeHistoryState;
myDocument.selection.deselect();
if (as != myDocument.activeHistoryState) {
res = true;
myDocument.activeHistoryState = as;
if (res == true) {
// set to pixels;
var originalRulerUnits = app.preferences.rulerUnits;
app.preferences.rulerUnits = Units.PIXELS;
// get  quickmask’s histogram for 0 and 255;
myDocument.quickMaskMode = true;
var theChannel = myDocument.channels[myDocument.channels.length - 1];
var white = theChannel.histogram[255];
var black = theChannel.histogram[0];
var total = myDocument.width * myDocument.height;
myDocument.activeHistoryState = myDocument.historyStates[myDocument.historyStates.length - 2];
if (white + black == total) {
alert ((white/total*100)+" percent\nselected pixels: "+white+" \nunselected pixels: "+black)
else {
alert ("the selections seems to be anti-aliased")
// reset;
app.preferences.rulerUnits = originalRulerUnits;