What dye do I need to buy to dye a dress of mine?

I'm happy you want to go from light colored to dark. That automatically makes your project easier. However,, there's still a couple problems. The Ritz Dye website has plenty of information about choosing dyes for your project,, using dyes and what can and can not be dyed. As a general rule you choose dye based on the dominant  fiber -in your dress it's rayon. Rayon can be dyed but the color will not be as bright or true as the package or bottle color. Additionally you will need twice as much black dye as any other color. Polyester doesn't dye and your dress might still end up as a grey and not black. There are polyester dyes available that no longer require boiling for hours. 

Have you successfully washed this dress or is it dry clean only? If it's washable you can dye it. If it's dry clean only then you can't dye it. Some rayon shrinks when wet but if the dress was washed in the past then dyeing is okay.

Check the Rit Dye website for more information.  It will help you learn and understand a bit more about dye and how to choose dyes for your fabric. Dyeing is a gamble and honestly, there's always a chance you could ruin what you are attempting to dye. If you are willing to gamble then go for it.

Black dye                            

First, rayon, polyester, and spandex are all highly resistant to aftermarket dyes, just as they are highly stain resistant. Second, aftermarket dyes requires using extremely hot water, but washing rayon and spandex clothes in hot water ruins them.

Basically, I'm telling you that you are more than welcome to try and dye that dress, but only do so if you don't mind losing the dress, if it's not something you'd ever wear the color that it is, light green, because there's a very, very, very strong chance, an almost certain chance, that you will ruin the dress if you try and dye it. What you will end up with is a dress that is unevenly colored, not true black, misshapen and shrunken in parts and not in other parts, and with a fabric that has lost its original texture but is permanently wrinkled and fuzzy looking.