Overglazes / Enamels / China Paints
These are all different names for glazes that are applied on top of the main glaze body, often applied with multiple firings as different glazes have different firing temperatures - in this case the highest firing temperature glaze is applied first, then working down the temperature scale.
These glazes use volatile oils instead of water, which results in a significant amount of organic matter being burned out during the firing. This requires a well ventilated kiln, either using a kiln vent, or firing with the bungs open, and possibly the lid wedged open. A slow firing helps avoid glaze defects from the organics burning out, and a wider spacing between pieces also helps by increasing the amount of air in relation to the area of the pots.
If underfired, the surface will tend to be rough and the colours dull. If overfired, the surface may be excessively glossy, and the colours lost or faded. Some colours may vary noticeably with variations in the firing temperature, making exact colour matches between firings, or even between different parts of the kiln, difficult to achieve.