Basic Answer
Prophet Muhammad's (God bless him and grant him peace) used a flag that was plain green, the color of Heaven.
The Saudi Flag has the words, "La illaha il-Allah" ("There is no god other than God") in white, on a green field. White might stand for purity or simplicity.
Discussion
It is interesting to note the history of the Eastern Pagan religious Flag in conjunction with Islam and The Turkish Flag
The Pagan Moon God Ilah's symbol was originally of the Eclipse Phase of the moon.
The Islamic Flag Demonstrates the phenomina "Red Eclipse" with a White halo representing the light of the horizon of a full Eclipse,
The background colour of the flag of Islam (green) is an adaptation of the Pagan Earth and Sea Goddess Ashera which connotes fertility mainly of the land allowing growth.
However, the Qu'ran wrote Ashera out of the Myth's, not only Ashera, but, Ilah's 3 children by her and changed the Pagan flag to the classic Turkish flag we know as the Islamic symbol today.
With the representation of the day that Muhammad's son died (the day of the Red Eclipse). Which was commanded by Muhammad not to be seen as a sign of God and not to be worshipped, however, the flag remains a symbol of this message lost in numerous pages of the Qu'ran
The red Background represents the shadow and death that occurred on that fateful day White event horizon is the sun peaking round the edges of the darkness and the star shape is the light just breaking through from the other side.
When measured with a circle you can see that the star actually fits perfectly along the edge of the circle, which is pretty solid proof that it isn't a crescent moon and a random star but the 'Red Eclipse'.
Butterflies symbolize a rebirth in Islam. In Islam dreaming about a butterfly is thought to symbolize that a person is near the end of their life.
The colour red means luck and regeneration in China. that is why it is used allot in Chinese festival's example: Chinese new year...)
RED!
Violet symbolises your face
Red is the colour for communism.
Red is the colour for blood, or love, or maybe could be seen as hope. Also, the little girl is wearing a red coat, like Red Riding Hood... So she is vulnerable, and she is being preyed upon by a wolf (the Nazis)
It symbolize Islam as Malaysia official religion.
There is no symbol especially prescribed in Islam. Mostly Crescent and Star are used as symbols. Both symbolize that Islam is a Heavenly Religion.
Red roses symbolize love, while white roses symbolize purity.
The colour of the gullet is red as our blood is red in colour.
Blood red is a vivid deep red colour, similar to the colour of blood.
Red and green can symbolize Christmas.