The Magen David is not an ancient Jewish symbol nor a religious one like the cross. It originated in Bohemia around 500 years ago. There is absolutely no archaeological evidence of David's existence. I am not suggesting that he did not exist, just that there is as yet nothing extra-biblical. Therefore it would be impossible to pin the Magen David on him.
Blues can be very confusing.
In Smith 1975 you will find that Israel and Greece both use a fairly dark blue, though they are slightly different from each other. The Israeli blue is as dark as any of the Blue Ensigns shown in the book, and the Greek flag is of a very slightly lighter shade. They both stand in sharp contrast to the light blue of Argentina , Botswana , Fiji , etc. The blue colour in Israel's flag is definitely dark. Some time ago I got a folder on national symbols from the Embassy of Israel in Washington.