The term "flaw" is correct; it defines the difference in a stamp to that of others in its category.
Having some spots on your skin doesn't mean that you are flawed
Tell that to a supermodel. If a customer buys a china cup and notices the handle slightly separated that is a flaw, it went through production and into a retail store for sale, if it were detected while being prepared for the distributor, it would be seen as unacceptable and destroyed.
A flaw is a variety not detected and has made its way to the post office counter; a printers waste was detected before issuance, placed in a bin and sent to the furnace, instead of completing its destiny from the printing press to the furnace it somehow left the premises by the back door and ended up, not on a post office counter but directly into the hands of a stamp dealer, such was the case of the 1940s material stolen from the Note Printing Branch; today these formerly stolen items are still not referred to as varieties (flawed), but for what they are - formerly stolen material from the Note Printing Branch,
Any philatelist with common sense will realize we have two variants here, no varieties
Variety is plural for variant, though "variety" is the preferred term to identify one or more errors today. And all philatelists today will call their special stamp a variety, not a variant.
Whenever you try to make lists of the (main) streams in stamp production, you will find that most shades have no systematical usage for the printer nor for the post office. It does occur that the Post Office decides that the colours are too heavy, too dark and want to have lighter, brighter colours. Then we don't speak of shades. It all boils down the conscious decisions by the printers or the post office to make changes.
What happens if the doctor blade removes too much ink or not enough and leaves an unintentional shade and that sheet is undetected and sent out with other sheets for distribution, or if a sheet not intended for distribution but was accidentally added to sheets for distribution and ended up at the post office counter.
If stamps that have errors of any kind, or have a shade change (more than one variation of the same colour) intentionally or not, they are known as varieties.
The 1982 Humpback whale on the left is the official design for distribution, the same stamp on the right was rejected and not intended for distribution; but through a mistake at the printers a sheet of the rejected design was included with the approved design.
This rejected sheet was eventually bought over the counter and later detected by a stamp dealer and officially referred to as a major variety (otherwords, a major variation of the official design}; if this sheet left the printers via the back door it would be referred to as printer's waste.