When you're running fuels with castor oil as a major part of the lubrication package, you're going to get some of it cooked onto the engine's parts. It's normal. Eventually, enough of the residue will build up to the point that it impedes the engine's running. Therefore, you have to clean the engine from time-to-time.

The anti-freeze system works well. You use an old crock-pot on the 'low' setting. Disassemble the engine and soak the parts overnight. This must be done in a well-ventilated area, and not in the house. You don't want to breathe the fumes.

Another way to clean the engine is to use hot, soapy water (dish detergent is good), and scrub all the parts with four-ought ('0000') steel wool. You can even use an SOS pad. Rinse in water as hot as you can stand with the bare hands, shake off, air dry, lube, and reassemble.

The color of the residue you find on the top of the piston or in the cylinder head depends upon how the engine was run. If it was on the rich side, you can most certainly get a black carbon buildup. Some of it may be soft and flaky. It scrapes off very easily. Some may be hard and black. That's what the steel wool is for. Brown varnish is just castor that hasn't been cooked enough to carbonize.

The black residue that signifies metal particles coming from the engine is the oil residue that's emitted when the engine's running. If the oil coming from the muffler is black, and has minute shiny specks, then you're getting metal particles. The engine is 'eating' itself. If the oil is black, or very dark, and there are no metal particles in the exhaust, then it's likely just oil that's being 'cooked' in the muffler, which does retain heat. There's more than just the color of the residue involved.