Color Brilliance

Hopefully someone can correct anything I get wrong here. The picture is a little dark, but it looks like a level 4 brown shade (can't tell if ashy or not - possibly ashy)

This is a 4a in Ion Colour Brilliance

Since you're blond, you likely had to bleach in order to get there? If so, be very careful in going back to natural. I recently made a similar journey and spent a lot of money on tints and colour removers because every time I tried to cover my blond, I got this greenish-grey tinge where there was no pigment left in my hair. If you have some yellow/orange or red tones, you should be fine. If not, you may have to pre-pigment. A 4 has a red pigment base, so you could get away with an ammonia-free copper blonde and then go over with a level 4.

I have read that when going darker it's best to aim warmer because ashy tones can really dull out when going over blond. If you would rather not go too warm, choose a 4N over the red. Every advice column I read said to steer clear from ash.

Always strand test!

As for developer, mixing brands shouldn't matter, but I guess there's the obvious brands are always formulated to work best within their own brands, but I suspect developer is developer. I was using salon care with ion for a long while until I needed a sensitive scalp one, so I picked up the ion one in 20.

Lastly! When going darker, you really don't need 20. 10 is preferable because it is least damaging and deposits tone, which is all you need to do. 20vol provides the lift one needs when going dark to light (approx 1-2 shades)(box dyes typically provide 20vol)... 30 lifts further. 40 shouldn't be used outside a salon.

Thanks so much for the advice! The blond picture is what it is now. I'm also posting a picture of when it was more reddish. Do you like the dark brown or the reddish brown? Do you think I should go with 4RV? I noticed that 4A is ashy. Also do you think 3 is too dark?

Current hair color

Permanent ash browns can show up green over a blonde base, so you will probably find the warmer colour easier (though they may show more red tones than they would on natural hair). Permanent colours usually fade easily over bleach too. When I went back dark I already had a ginger direct dye (Adore) on my hair but I used a reddish brown( Wella Koleston 5/4 I think ) but it still fades to a golden toned brown/blonde at the ends where the bleach was and the ginger direct dye must have faded. I use Adore browns to keep it dark.