[DRG]

I used flags of the world ( FOTW ) as a reference for all the national flags

http://www.crwflags.com/fotw/flags/gr.html
http://www.crwflags.com/fotw/flags/gr_evna2.html

The two Greek national flags
According to the book Hellenic flags [k7k97], Greece had from 1828 to 1970 had two national flags. The cross and stripes flag was used outside the country, including at the coast, whereas the plain cross flag was used inside.
The flag used at sea and on the coast had nine horizontal stripes, in turn blue and white, and a white cross on a blue square field in canton. This flag is today the Greek national flag.
The former flag of the merchant navy was abolished in 1828, when it was decided that merchant ships would henceforth use the same flag as the Navy.

They also say......

The striped flag has been in use since 1822,

and

Shade of the Greek flag
There is no officially prescribed shade of blue for the Greek flag (photo) in the 1978 Law. The 1970 Law that abolished the plain cross flag did not specify a particular shade either, but it did provide that all flags should conform with "prototype" flags lodged with two government departments.
The shade of blue on the prototypes was, probably, very dark hence the very dark shade of flags made in 1970-1975. A lighter shade of blue is used nowadays (by convention, presumably) but still not as light as United Nations blue.

All of which is the reason we use the same one from 1930-2025