Identifying Excel Sheet cell color code using XLRD package

This function returns cell background's rgb value in tuple.

def getBGColor(book, sheet, row, col):
    xfx = sheet.cell_xf_index(row, col)
    xf = book.xf_list[xfx]
    bgx = xf.background.pattern_colour_index
    pattern_colour = book.colour_map[bgx]

    #Actually, despite the name, the background colour is not the background colour.
    #background_colour_index = xf.background.background_colour_index
    #background_colour = book.colour_map[background_colour_index]

    return pattern_colour

Here is one way to handle this:

import xlrd
book = xlrd.open_workbook("sample.xls", formatting_info=True)
sheets = book.sheet_names()
print "sheets are:", sheets
for index, sh in enumerate(sheets):
    sheet = book.sheet_by_index(index)
    print "Sheet:", sheet.name
    rows, cols = sheet.nrows, sheet.ncols
    print "Number of rows: %s   Number of cols: %s" % (rows, cols)
    for row in range(rows):
        for col in range(cols):
            print "row, col is:", row+1, col+1,
            thecell = sheet.cell(row, col)      
            # could get 'dump', 'value', 'xf_index'
            print thecell.value,
            xfx = sheet.cell_xf_index(row, col)
            xf = book.xf_list[xfx]
            bgx = xf.background.pattern_colour_index
            print bgx

More info on the Python-Excel Google Group.

The Solution suggested by JMax works only for xls file, not for xlsx file. This raises a NotImplementedError: formatting_info=True not yet implemented. Xlrd library is still not updated to work for xlsx files. So you have to Save As and change the format every time which may not work for you.
Here is a solution for xlsx files using openpyxl library. A2 is the cell whose color code we need to find out.

import openpyxl
from openpyxl import load_workbook
excel_file = 'color_codes.xlsx' 
wb = load_workbook(excel_file, data_only = True)
sh = wb['Sheet1']
color_in_hex = sh['A2'].fill.start_color.index # this gives you Hexadecimal value of the color
print ('HEX =',color_in_hex) 
print('RGB =', tuple(int(color_in_hex[i:i+2], 16) for i in (0, 2, 4))) # Color in RGB