In the UK, it is available direct from the author price £49 (including postage). It is also stocked by Amazon and other on-line booksellers, and is available in some bookshops.
Your local bookshop should be able to order it for you; the bibliographic information is given below:
Author: Phil Green
Title: Understanding Digital Color 2nd edition
Publisher: GATFPress, Sewickley, PA
Date: 1999
Library of Congress Catalog Card Number: 99-64453
ISBN: 0-88362-233-5
Quantity discounts are available. Contact Peter Oresick at GATF (800/910-GATF in the US; 412 741 6860 from outside the US)