I heard that veggie dyes are good for you...such as Manic Panic or Special Effects used over either dyed or natural hair.  I was wondering if these are safe for hair and to use over hair? For example, if I wanted to use a bright blue on my dark hair for a blue black color, would that work? I don't want to ever bleach my hair again and to grow out my natural color, but crave a bit of unnatural glow/sheen such as blue, purple or ruby without damage.

My natural hair color is dark brown. I used to use these colors. What I found was that a color like purple or blue made my hair look blackish without any appearance of purple, except on the gray sections. When I used pink, red, or orange, it was much more apparent.

If you're just using the color (no bleach), it will wash out after a week or two, whether you like it or not.

The veggie coloring is not permanent - it just coats the hair strand.  The color will be more obvious on lighter strands.  Some people have reported that the color lasts longer when used over recently chemically processed hair (coloring or perms), as the hair is more porous.