I do not see it documented in the User Guide, but the lock and key colors in the Security column of inSSIDer seem to correspond to the following:
Green Closed Lock = WPA2 Personal
Green Key = WPA2 Enterprise
Orange Closed Lock = WPA Personal
Orange Key = WPA Enterprise
Yellow Open Lock = No security/WEP
If a network has more than one security type (WPA personal and WPA2 personal, for example) then the color will correspond to the lowest security protocol.
Are there other colors used besides the above?