I just managed to get a LiFX A19 light to work with Homekit after a tedious hour or so. Eventually I found a tip to change the name of the device during wifi setup (wifi settimgs in iOS). After that the LiFX app found the light and so did Homekit.
Another tip is to reset the light before trying to connect to the network. With the A19 this involves switching it off and on 5 times - it should then change color a few times before going white and is ready for set-up.
Another tip is to reset the light before trying to connect to the network. With the A19 this involves switching it off and on 5 times - it should then change color a few times before going white and is ready for set-up.