The developer guide indicated that the 'sendContext' command needs to be used to send context to an Eikon colour channel. It requires a 'targetInstanceId' to identify the channel. When color channels are linked, they do NOT return an instance Id. Presumably this should be a channel #, such as 0 for the red channel.
When a color channel # is passed as a 'targetInstanceId', data.isSuccess indicating that it worked, but nothing is changed in Eikon - objects set to that channel have NOT changed context.
Once the channel is linked, you need to use the command 'contextChanged', and pass the 'channelId'.
However, although the documentation doesn't say so, only ONE colour channel can be linked at a given moment. You can attempt to join more than one channel, and get a successful result, but when you try to use it, it will not function correctly. According to SxS product development, it was never intended to allow more than one channel to be opened at the same time.
As a work-around, you can open one channel, send a command, and then close it.
Then you open a different channel, send a command, then close that channel.
In my experience, this works.