Try this:
/local <scale 1.50><color #00ff00><bordercolor #ffff00><bgcolor #9f00ff>I just leveled, kneel before me!!!
Explanation --
scale: can be a decimal size from 0.5 to 1.5. 1.0 is regular size.
color: color of text. Use RGB codes from HTML HEX colors.
bordercolor: color of bubble border. Same RGB codes as "color".
bgcolor: bubble background color. Same RGB codes as "color".
HTML Hex Colors uses six hexadecimal colors for Red, Green and Blue in that order. 00 is no color, FF is full blast. Every number in between is a tint of that color.
00, 33, 66, 99, CC, FF are example steps for one of the colors.
The colors are Red, Green, Blue in that order, so:
#FF0000 would be Red (Red: FF, Green: 00, Blue: 00)
#00FFFF would be Cyan (Red: 00, Green: FF, Blue: FF)
#FFFF00 would be Yellow (Red: FF, Green: FF, Blue: 00)
Or you can
use a color picker to do this for you.
I don't know how Optionset works, but that might be a clue in figuring it out.