Sorry for the slow response; I don't notice your message for a few days.
I was using GraphicConverter on the Mac to draw the final versions of the tiles; this program allows you to save a palette from one file and load it in another (I'm happy to explain how, if you're interested).
Otherwise, you could probably make a copy of 32x32.png, and resize the document to a more manageable size for editing/drawing. This way, your palette matches the 32x32.png file, so you can use compatible colors from the beginning; no colors will be lost if you copy a new tile to the 32x32.png file.
When I drew the first few tiles I made I didn't realize all this, so I had to redo the coloring by hand to fit the new palette after I pasted my tiles into 32x32.png. After the first few tiels, I started with the correct palette, so I only had to color the tiles once.
"I noticed the color degradation as well. My solution was to copy the palette from 32x32.png to my file, and then redo/fine tune the coloring based on this palette. If you copy the palette first, then you can save yourself some time."
Could you give me a bit more detail how to do this. I don't completely understand. TYIA