It's normal for the oil to look that way. Sorry to say but when you buy a used differential it's on You to check the oil level change it too. We live and learn its all good.Small metal shavings. The old fluid looked pretty silvery in color vs black
On my machine I changed all fluids at 100 miles, the front diff oil was silvery with metal fuzz. That's wear from the break in process which is normal. It's important to change fluids in the beginning to get the wear metals out, after that It's much less.