What used to work for me in the beginning when I started:
STEPS FOR CONDITIONING WITH HENNA (no drastic color change)
1. Use cooler water to mix the powders
2. Add Amla &/Or Shikakai Powders to your henna (they have conditioning properties, as do Bhringraj and Bhrami powders)
3. Do not a citric acid based liquid or known terp (ACV, tea tree oil, lavender oil, etc).
4. Do not leave in the hair longer than an hour (the longer you leave it in, the more time the henna will have for color release). The terps HELP to speed the color release, not using them will only make the process take a lot longer so the sooner you use it after mixing, the better.
5. Don't use extra heat during the conditioning process (body heat will be fine).
I still do this when I want just conditioning and no/less color or, I do a henna gloss. I prefer doing this though because I've really grown to like the results from the henna better than any of the conditioners I have in my stash. Mixing my henna w/a conditioner at this point feels like I'm wasting my precious henna.
Side Note:
For the ladies who have a hard time rinsing after your henna application, I rinse my hair in the shower, trying to do so in the sink made for a HUGE mess everywhere and it took forever. I just take the spray cleaner with me and clean after I finish showering. The henna doesn't stain the tile/grout. Oh and no, I don't end up red all over cause of the henna!
ETA: Thank you CarmelDiva and Kitchen_tician!