G'day
Lighter or darker coloured gold is generally due to having a higher content of either copper or silver, both of which are closely associated with gold, but also as said by Deepseeker some nuggets can be iron stained or coated with a black manganese oxide, or can even stained by some other iron oxides in the ground, the coatings are generally easy to remove with an acid bath, but unless you are aware that nuggets can come out of the ground this way in some places then it would be easy to think that it was just some chunk of steel or lead and thrown away.
My first ever nugget that I still have to this day was completely coated in a white calcrete type coating, when I picked it up I was not sure that is was gold and as I wasn't sure so I just pocketed it to have a better look at it later, the second swing over the same spot near the base of some old mine workings produced a rusty wood screw so I wasn't convinced, later in camp I used my knife to scratch the edge of it and instantly saw the gold, been hooked ever since.
I have also encountered a spot where I found many black nuggets, heavy ironstone ground and very volcanic looking rock, the first one I found I was in doubt that it even was a nugget, I tried to clean it a bit to see if it was gold and stuck it in my mouth and it tasted really bad, so I don't do that anymore, ( and this is a very bad thing to do as it can also make you very sick ) but the weight convinced me it was gold so I kept it, I found lots more and once cleaned in an acid bath the nuggets were bright and very smooth.
Many people have also thrown specimen nuggets away as well, as some unless they have visible gold can appear just like a hot rock, probably did that myself in the early days, what I do now is put it on a flat rock and give it a belt with the flat end of my pick, most hot rocks if they are light will just disintegrate, but if they are heavy and more solid I just pocket them and have a better look at them later when in camp, but I never throw them away until I am sure there's no gold in it as I have found gold in all sorts of stone from quartz, ironstone, brown rock and everything in between, and many of these rocks can also be very stained with oxides as well so they are always worth investigating just in case.
cheers
stayyerAU