The green code is simply a condition that you have to satisfy before you can check into your flight. When flying from HK/Taiwan there is no such requirement at check-in. Otherwise you must still satisfy the same requirements as in pre-Covid times, in other words a valid Chinese visa/RP before you are allowed to check into a flight to China from HK/Taiwan. Your residency in Taiwan/HK is irrelevant for the check-in process. That is, even if you are currently in HK or Taiwan with a visitor's visa, you are still allowed to fly to China. Of course, if you're not a resident of Taiwan/HK, you can't currently enter either of them from abroad (unless you can obtain an exemption, e.g., through a resident spouse).

If you are travelling from HK there is also the option of a land border crossing, either at Shenzhen Bay or via the Zhuhai bridge.