If you know how to henna (PROPERLY!!) your hair with thank you -- but beware, henna is a wand and a sword -- if you don't know what you are doing, your hair will be jacked; on the other hand, once you master the technique/blend, it makes your hair smooth AND strong and greys will be a thing of the past.
Commercial Hair Dyes? I actually had better experiences with going lighter than darker. Clairol Texture and Tones blondes always did well in my hair and seemed to "texturize" it a bit. (A lot of debate on whether colors can "straighten;" in my experiences, going lighter did.)
Now, for the evil commercial darks (IMO) -- DESTROYED a patch of my hair (where the part is when I wore weaves -- yup, front and center). Tried Demi (Demi!!!!!) dark brown or black and I swear it messed up my root structure -- years later, that patch still grows in slower and more brittle than the rest of my hair.
Summary -- hennas and commercial lightening (if you do your homework, read relevant threads, etc.) are fine; dark commercial dyes (i.e., anything stronger than a semi permanent rinse) - stay away.
Oh and just remembered this -- if you do a semi (never demi, IMO) permanent dark rinse, apply when conditioner is already in your hair. A stylist told me this and it really makes a difference -- you notice that hair is softer/shinier and the color still takes for a few weeks depending on how often you wet your hair.