Is it ok for hair dye/bleach to be put directly on scalp?

[Nickie]

I get my hair highlighted and dyed,and I'm already on propecia/rogaine, from dark to blonde, and I have long hair so it really helps make me look better, more like a Matthew McConaghey type. However they sometimes put the hair dye, directly on the scalp especially in the front to touch up my roots, and they claim it's completely safe, and the dyes they use have improved in technology and won't harm my hair.

Is this true, or could it be damaging my follicles? Or does the real damage occur from the inside really?

I'm thinking that this stuff can only irritate the scalp itself not particularly the hair follicle inside.

Thanks!