is it possible to get a humbrol shade card

The Model Master Modeler's Techical Guide has a color chart for thier colors.
Plus it has cross reference charts for exact or close matches of enamels and acrylics
from other manufacturers when available. There are some Humbrol and Xtracolor
matches listed. Stock # 2020C.

Here is a good site with color charts and comparisons:

http://www.ipmsstockholm.org/