Identifying used KGVI stamps from Hong Kong can be really challenging.
First, there were many printings with variations in the colors and in some cases perforation and chalk or substitute (ordinary) paper.
None of the catalogs list all the printings (luckily), so you might have two unused stamps that vary in color but are still the same catalog listing. If those stamps are used, the color might have also been affected by exposure in the mail.
So with that in mind, I have scans from my collection that might help you. They are based on the Commonwealth Catalogue (Murray Payne), but I also included the Stanley Gibbons and Scott numbers on the page.
The 20c has two listings in both Commonwealth and Gibbons, but there were six printings from 1948 until 1952. Since you asked about Gibbons I will use their descriptions: SG 148 - Scarlet-Vermilion, and SG 148a - Rose-Red. If you have both stamps, the Rose-Red one will be a paler color compared to SG 148.
The 50c is more complicated because it was in use from 1938 until the early 1950's. There were 13 printings from 1938 until 1953. Commonwealth and Gibbons both list four of them. Here are the Gibbons descriptions:
SG 153 - Purple - Perf 14 - Unused stamps typically have a creamy gum and the paper appears thin when viewed against black paper compared to the other printings.
SG 153a - Deep Magenta - Perf 14.5 x 14 - forget the color. Just check the perfs. This is one of the Bradbury, Wilkinson printings after De La Rue was bombed.
SG 153b - Reddish-Purple - Perf 14 - Unused stamps tend to have a clear gum. There is more than one color shade for this stamp. Identify the other Perf 14 issues first and if it is not one of them, it is probably this one.
SG 153c - Bright Purple - Perf 14 - Chalk Paper. If you can't identify the chalk paper look at the back. The gum might tend to be whiter than the other issues and the paper will tend to appear thicker with an opaque watermark that is not as visible as the other issues. There were eight printings on chalk paper, so you will find some variation in colors if you have multiple issues.
Shown below are two scans showing the stamps from my collection so you have some reference. I had to rework the numbers because both Commonwealth and Gibbons changed the numbers from what they were when I created the page back in the 1980's. I hope this helps.