I personally think that a red>orange/yellow, blue>teal, or green>yellow would flow better (being the different colors that blend into each other in the sig) as opposed to the green>red that you're currently using. These would clash more with the ava, but fit better to the sig.
Other alternatives would be white>black for the mask, green>black for the clothes, or yellow>grey like the text. Pale green>grey/rust color would fit the ava better imo.
To get the exact codes would simply be to open up photoshop, paste the image in, and use the eyedropper to get the exact color code. You most likely won't get it perfect on your first pick, but that's just trial and error.
(An alternative would be to explore color codes without looking directly at the signature. There's tons of sites with the colors and their codes listed.)
Also keep in mind that the rainbow orb looks different on different browsers, so you should provide that information too.
Step 1 would be to decide what colors you actually want to see.