You should be able to do this via the multi renderer you should have a line like this...
XYMultipleSeriesRenderer mRenderer = new XYMultipleSeriesRenderer();
You can then use mRenderer.setylabelscolor and .setxlabelcolor but once you enter the .set your development environment should show you all the set commands.
If not the docs are good
XYMultipleSeriesRenderer[
^]
You might also want to take a look at
Android tutorial for achartengine[
^]
/Darren