Like from code, you could loop through each pixel, check if it is the color to be replaced, and if so set that pixel to new color. There might be some image software out there that does this. But really is trivial to code.

How big are these images? And I did a Google search and found this: http://docs.oracle.com/javase/6/docs/api/java/awt/image/RGBImageFilter.html Which I will be reading more to see what can be done with it

Do you mean '20 times per act cycle' (which would be impossible without the use of delays) or 'the image will be changing colors about 20 or more times in a row, one change per act'? If the latter, I would have the images of all colors pre-made with references in fields and just set the image accordingly each cycle.

its an editor with cells the cells can be changen from lets say 5 px to 100 px or something so the pictures can be 5*5 or 100*100, if the screen is 500*500 big, there is with cell size of 50 space for 10*10 pictures so there can be many of these each cycle

Will the sum of these cells create a multi-colored grid? or will all cells have the same color at any given time? Is each cell an Actor object?

With your last post, I would have to say I totally mis-interpreted what you are trying to do. Well, previous to last.

this is the way it looks like at the moment: http://www.fotos-hochladen.net/view/unbenanntgur9oykw61.jpg but the objects (which will get more different shapes) are all gray but they should get colored

What changes their color? What event takes place that they need to change their color? Are they really changing colors each act cycle, like fading from one color to another or what?