Official policy, as other have stated is white paper, but I use anything I can find and use, cover them fully with clear shipping tape (sometimes 2+ layers) and have had no issues.
If colored paper, it's usually only lighter, pastel colors, but have used darker colors like red a few times with no issues.
I use old paper I have from the 1970's that has yellowed and browned, yellow legal pad paper, clean sides of old printouts (like back of a single sheet 2017 calendar I printed last year and last year's USPS and FedEx printed price sheets), office printer booboos (as long as nothing personally identifiable or than might offend is there), clean back side of junk mail pages, and pieces cut from the backs of junkmail, greeting card, and other envelopes when they are big enough (like 6x8 or bigger).
Pretty much anything I can shove in my laser printer sheet feeder gets used as label stock. I use clean 20lb bond when I'm out of recycle paper or I have an inkling it would be wise to do so based on buyer and purchase/package type.
Zero problems from USPS, FedEx, or buyers to date.
I also use my old wall calendar pages as item wrapping sometimes, and the fronts of greeting cards as backing stiffeners/protectors in poly and padded bubble mailers when appropriate. Waste nothink.
