If the police spot Carl spraying a tag, a Wanted Level will be applied, which should be noted with tags located in high-traffic areas. This is because the spray can is counted as a weapon, and "shooting" a weapon will cause you to attain a wanted star when the police see you, though a "real-world" perspective can also be applied in that spraying graffiti is generally considered an illegal act by itself.
A tag is not considered completely sprayed until you see the on-screen message confirming it and you hear the comfirmation sound.
Spraying over any tags that will be used before the mission, Tagging Up Turf will result in the tag being reset and you must respray every tag in that mission again.
Before you unlock the spray can item in the Johnson house, you can find one on the rooftops of the shops directly behind the house, which can be obtained by jumping onto the low wall in the alley between the Johnson house and Ryder's house.
The Varrios Los Aztecas gang tag is orange (like the Vagos'), however, their gang color is turquoise.
From the start of the game up until Sweet's arrest in The Green Sabre, the Temple area is territory of the Temple Drive Families. It is only after that mission that Temple gets occupied by the Ballas, so it must have been a mistake as to why 'Temple Drive Ballas' tags are present right at the beginning.