You firstly define the gangzone area:
https://sampwiki.blast.hk/wiki/GangZoneCreate
Then, you set the color:
https://sampwiki.blast.hk/wiki/GangZoneShowForPlayer
This will help you on how much intense color you want:
https://sampwiki.blast.hk/wiki/Colour_Em...ransparency.29
That's all volks.
P.S.1. Next time use ******.
P.S.2. Use the GangZoneForPlayer under "public OnPlayerSpawn(playerid)"
Use the GangZoneCreate under "public OnGameModeInit()"