How to bring out natural red undertones?

A lemon juice rinse might help. Henna might help too but it might make it darker.

I was going to say henna, although you may not get the shade you want.

You could go in a tanning bed and protect your skin if you are concerned about sun exposure. That would give you the most accurate color probably. Otherwise, I would suggest going to a professional a couple months after the fall sets in where she can still see the color you are trying to acheive and do her best to mimic it.

Tressa Watercolors Shampoo. It’s pretty intense (leaves a pink stain on your hands).
I mix it 1:1 with my regular shampoo and use it daily.

I have the opposite problem. I do not want my red
undertones to show and every time i get my hair
dyed the red comes out (i have a grandmother with red hair)
It shouldnt be rocket science! what do you mix with a med
brown hair dye and not come up with red?