the color of the phone affects this, the ocean blue with yellow stylus is set to write in yellow, while the purple phone writes screen off in purple.
I tried the pen of one on the other and it's not pen based.
If you go to advanced features spend settings then screen off memo you can turn off the "write in signature color". This returns it to the off white previous versions used which snote converts to black.
This is a total oversite by Samsung, I expect it will be an easy software update fix. Yellow on white is readable but stupid. Until then turn off this "feature" on the blue phones.