I can only speak to open pvp servers however,
green "dove mode" - you can only attack people that have attacked you. You will not receive pz block.
white "party/guild defense" - you can attack and open a pvp situation on people who have attacked a party or guild member. You will not receive pz block.
yellow "skull bash" - You can attack any skulls. You will receive pz block when attacking or opening a pvp situation on players who have not attacked a party or guild member.
red "aggressive" - You will open a pvp situation on any player who walks into you, is in the area of effect of a rune or steps on a field cast in this mode. This mode should almost never be used due the the high possibility of a red skull.
All field runes and bombs placed or creatures summoned while in yellow or red mode will remain in these modes for the duration of their existence. For example, summoning a fire elemental that shoots a fire field, any player that steps on that field can suicide and give you an unjustified kill.