The palette I use for Oil painting is excellent, but not intended originally as a palette at all.
It is in fact an Ikea glass shelf with a grey toned back. This works perfectly for mixing paint, as it gives a mid-tone and allows me to choose how the mixing goes, either darker or lighter.
The best part, is the clean up of this palette. I can leave Oil paint on there for days/weeks and when it comes to cleaning, scrape some with a wide-scraper, or use some methylated spirit which successfully dissolves the oil paint.
As you see, this time I had a Very Messy Palette!
If you click on the photo, you'll see my Clean Up with a difference.