sometmes i need to set brightness immediately back to default (= 0)

What steps will reproduce the problem?

try this:

var _tg:TweensyGroup = new TweensyGroup(); _tg.brightnessTo(MYOBJECT, 0, 0, null, 0);

What is the expected output? What do you see instead?

> MYOBJECT disappears, after function has been started

What version of the product are you using? On what operating system?

flash cs5/ os: win7, build7600.x

Please provide any additional information below.

none

That's because the ease functions return NaN, due to division by Zero. You can either set the duration in your call to a very small value (e.g. 0.001) or you can modify TweensyTimeline.as:

Replace the two occurrences of: var position : Number = ease.apply(null, args); by: var position : Number = _duration!=0 ? ease.apply(null, args) : 1;