Used a bleach shampoo over colored medium brown hair, natural color is dark brown and now my hair is a reddish brown kind of brassy. I was told I should use a light ash brown with a green base to counteract the red in my hair . I also have highlights in dark blonde. Is this information correct? What developer should I use 10, 20, or 30?

30 developer. The stronger the developer, the faster and more powerful it will lift out color. But be careful with bleaching your own hair, it'll cause severe damage, especially with a 30 developer.