see: http://nullege.com/codes/search/openpyxl.style.Color
example:
import openpyxl
from openpyxl.style import Color, Fill
from openpyxl.cell import get_column_letter
..
cell = sheet.cell(column=pt[0], row=pt[1])
cell.style.fill.fill_type = Fill.FILL_SOLID
cell.style.fill.start_color = Color(color)
cell.style.fill.end_color = Color(color)