Hi Arbor. If you go to your profile and click on "edit profile" you'll see the space to edit your avatar. Along the top, there are some tabs that show up as well. Click on the one that says "personal" and if you scroll down a little bit once you click on that there is a box for signature. Put whatever you want in it, then save.
What are the numbers I would use for a nice purple or brilliant green
I'm not sure which green or purple you'd like, so I will tell you how to get the number. The number is a six digit color number called a HEX number. All colors have HEX numbers or codes. They are listed with the # sign followed by the six digit number. So for example #000000 gives you black and #FFFFFF gives you white.
On this site, you can go to respond to a post and find the hex number for your signature that way. You go to the post and click on the little color wheel next to font size. You need to be in the "advance" editor to have the option.
Then click it and you'll get this cool box that appears. It asks you to choose a color. Using the little square, you can drag it around to the color you want. You can use both the slide bar to the right and little square to adjust it to the color of your choosing. On the right, it shows you the current color on the top of the square, and the previous color on the bottom. It also lists HEX: and 2fda20. That is your number.
Copy and paste that number into your signature code. Make sure it has a # sign in front of it. For example:
[FONT COLOR=#2FDA20 SIZE=7]YOUR TEXT HERE.[/FONT]
And that would give you this: YOUR TEXT HERE.
Alternatively, you can use color websites that display HEX codes for colors. There is one called Colorpicker.com which is basically the same thing, just off-site. There are also sites that have pre-made pallets with numbers, where you can find a color you like that way and use the number. It is up to you.