I have the 16th edition, published in 2011. The stamp illustrations are in black & white but the advertisements are in colour. I'm not sure if a revised edition has been issued since. Stanley Gibbons have the 16th edition in digital format but no edition at all in the catalogue section: https://www.stanleygibbons.com/shop...64&groupId=3
The 16th edition is the most current, although it is getting a bit long in the tooth and I think is sold out from Gibbons. A 17th edition is planned to be released around the time of London 2020 in May. There's talk that it might be issued in two volumes - one for the line engraved and one for the surface printed and embossed stamps. With each edition, the "specialists" try to get the listings revised to better reflect current research on the stamps and the differentiators.
If you can get a 16th edition at a reasonable price, I'd pick that one, as they were issued about every two years prior to the 16th, which makes other editions fairly old and not necessarily accurate.