Hi,
If you want to change the color of the annotation created using pen tool present in the annotations toolbar, that is possible.
After creating the annot tap on it. A box will appear surrounding the annot you have created.
Along with it you will see four options "Delete", "Thickness", "Color...", " Opacity...". Tap on Color option. A color palette will appear. Select the desired color.
From next time onwards the selected color will become the default color.
Thanks,
Ankit