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I don't think this is possible with IFTTT. I believe I did something similar with Yonomi a while back. Give it shot and report back. https://play.google.com/store/apps/details?id=com.yonomi&hl=en
I think you might be able to do this with The Yonomi app (I don't yet have a harmony but it is compatible). You can set up "routines" to control multiple devices, light colour/brightness can be set and routines can be triggered by you arriving home, leaving, other options. This should be possible.
Maybe Yonomi - Smart Home Automation but ratings aren't that good as they were once.
No problem! Just wanted to also add that since I made this post, I've actually done a little more research into smart home stuff. From my understanding, if you don't have bluetooth built into your receiver, you may actually not need Smart Things if you want to be able to control the outlet at the same time as the music mode.
If you get a wifi controlled smart plug (like this one), Alexa will be able to discover it over wifi without a hub like Smart Things. Once you get Harmony all set up and working with your setup, you can then get a phone app and Alexa skill called Yonomi. It basically discovers all your smart devices and lets you set up custom activities for them in the app, and lets them be discoverable by Alexa through the skill. If you then recreate all your Harmony activities within Yonomi as Yonomi activities (basically by making a new activity called whatever you want within Yonomi, that when invoked, activates the Harmony activity of your choosing), all your Harmony activities will then be treated as devices in the Alexa app.
This means that you can make a group within the Alexa app consisting of your Music Mode activity (the one made in Yonomi that calls the one made in Harmony to turn on the correct devices), as well as your smart plug. Then just name that group Music Mode (or whatever you want to call it). Then you can just say "Alexa, turn on Music Mode" and she'll power on the plug and have Harmony turn on the right stuff. At least this is how it works from what I've seen. Definitely a bit of work, especially if you're planning on making a lot of activities for Harmony, but arguably ultimately worth it since you don't have to drop the extra $100 for a Smart Things hub.
Now, if you plan on getting into the smart home scene down the road, Smart Things opens up a lot of neat things you can do. So if that's the case, it may still be worth the investment to get a Smart Things hub. Then you could get a wifi smart plug, or a plug compatible with smart things (one that works using Z-wave, for example). But hey, you know, just another option.