I think I would use a less frustrating platform if I were you. Why are the docs so bad??

Here's what I found: http://touchlua.wikia.com/wiki/Sys

It takes a table representation of a color...

Then I found http://hawwashsoft.proboards.com/thread/254/text-calculator

Which uses sys.setbgcolor like sys.setbgcolor(draw.black)

So I found the draw library on the wiki: http://touchlua.wikia.com/wiki/Draw

Looks like it takes a table like {r, g, b, a} where alpha is optional.

Seriously though, I would dump that app. Especially you're trying to learn Lua, it just adds burden to the learning process that TouchLua's docs are mostly nonexistent.