Demigirl-
Before we get into when the flag was made, let's talk about the colors.

White - neutrality and/or a questioning and shifting of gender.

Pink - The traditional color of femininity

Grey - the partial nature of the gender

Although these colors are not really "confirmed" most people believe these are what the colors represent.

It was made either on or before July 4th 2015 by Tumblr user Transrants.

Demiboy-

Blue - boy

White - agender or nonbinary gender

Grey - partial

The flag was also made by Transrants, but the original post has been deleted.

Demisexual-
No one really knows when and who made the flag. We just think it was made after the asexual flag after 2010. It shares the same colors which are based off of the AVEN logo.

Black - in both flags it represents a lack of sexual attraction

Purple - The ace-spec community

Grey - greysexual which demisexual is under

White - on the demisexual flag it means sexual attraction

Although we don't know who made the flag or when, we do know that one of the "alternate" flags was designed on August 8, 2018 by crimsxnflxwerz on Tumblr

Demisexual flag

Asexual flag