[Kostas']

Check again if return 1;

pawn Код:

public OnDialogResponse(playerid, dialogid, response, listitem, inputtext[])
{
    if(dialogid == 90) {
        if(response) {
            if(listitem == 0) {
                if(IsPlayerInAnyVehicle(playerid)) {
                    ChangeVehiclePaintjob(GetPlayerVehicleID(playerid), 0);
                }
                else return SendClientMessage(playerid, COLOR_RED, "[ERROR] {FFFFFF}You are not in a vehicle!");
            }
            if(listitem == 1) {
                if(IsPlayerInAnyVehicle(playerid)) {
                    ChangeVehiclePaintjob(GetPlayerVehicleID(playerid), 1);
                }
                else return SendClientMessage(playerid, COLOR_RED, "[ERROR] {FFFFFF}You are not in a vehicle!");
            }
            if(listitem == 2) {
                if(IsPlayerInAnyVehicle(playerid)) {
                    ChangeVehiclePaintjob(GetPlayerVehicleID(playerid), 2);
                }
                else return SendClientMessage(playerid, COLOR_RED, "[ERROR] {FFFFFF}You are not in a vehicle!");
            }
            if(listitem == 3) {
                if(IsPlayerInAnyVehicle(playerid)) {
                    ChangeVehiclePaintjob(GetPlayerVehicleID(playerid), 3);
                }
                else return SendClientMessage(playerid, COLOR_RED, "[ERROR] {FFFFFF}You are not in a vehicle!");
            }
        }
    }
    return 1;
}

If it still, change the Dialog id. Maybe there is same id at another dialog