Here some info, maybe its usefull.
Visitor Count - This value counts the number of eligible clan members that have visited the citadel since the last Build Tick. Each tier level in citadel will require increasing members to visit. If the value is red, there have not been enough eligible clan members visit. Yellow value means not enough to upgrade citadel, and a green value represents all requirements met to upkeep and upgrade.
Visitor Count - This value counts the number of eligible clan members that have visited the citadel since the last Build Tick. Each tier level in citadel will require increasing members to visit. If the value is red, there have not been enough eligible clan members visit. Yellow value means not enough to upgrade citadel, and a green value represents all requirements met to upkeep and upgrade.