When the solo button is green it's in Save Solo mode, meaning that it will still play when other tracks go into solo. Shift + Click the solo button to toggle it on any channel. See the reference manual section pasted below.
9.2.23 Save Solo
It is possible to place Console channels in Save Solo mode. When any channel in the Console is soloed, all channels with Save Solo engaged are also soloed, and all other channels are muted. To engage Save Solo on any channel, [Shift]+Click on its solo button in Console. The Solo button will be green when Save Solo is engaged.
Note that FX Channels have Save Solo engaged by default because effects may be critical to how soloed channels sound in the mix, so when any channel is soloed, you will probably want to hear the applicable FX Channel output as well.