Gasket makers go by many different names, such as form-a-gasket, RTV, Silicone, gasket material, and others. Some names refer to more specific types of gasket makers and are widely misused.
RTV silicone for making gaskets is usually sold in a tube, which contains a thick liquid that becomes solid like a soft rubber material after being exposed to the atmosphere. These often emit an odor when curing, because the chemicals that keep it from becoming solid are bonding with elements in the air. There are different formulations for various purposes, some of which are cross-compatible or may only differ based on patience and budget.
General/Soft RTV Silicone
Most formulations of blue silicone are designed to be quite soft when cured, sensor safe (always confirm that you are using this if necessary), and are typically used on parts where the temperature isn't too extreme. The soft nature of the formulation allows for better seals and the color allows for easier damage inspections.
General purpose silicone RTVs are often able to be packaged differently or in bulk due to their longer shelf life and higher sales levels. Quick cure time gasket makers are an example of this.
High Temperature RTV Silicone
Most red RTV silicone formulations are the high temperature resistance option, but not the highest rating. This silicone is known being the formula that produces the strongest odor when curing. Copper RTV silicone is a higher quality option, due to its higher temperature threshold, better thermal conductivity (due in part to the powdered copper metal within it), and slightly more rigid formula. The copper version can be used in some applications where red can't be used at all, such as on exhaust flanges.
Rigid RTV Silicone
Gray silicones are usually formulated to be more rigid when cured. This kind of silicone is necessary when two parts need a decently thick gasket but must also be torqued down tightly. Rigid silicone is also beneficial where lots of vibration will occur, or where twisting may take place.
Chemical-Resistant RTV Silicone
Generally, oil-resistant silicone gasket makers are black in color, but formulations for specific types of lubricants often take on various shades of grey. The version designed for use with gear oil is typically a light grey with traces of aluminium powder in it. Water pump/thermostat formulations resist various types of coolant and are usually light grey in color, but are quite soft, unlike most of the general-purpose grey options.
Clear Silicone RTV
Clear silicone is typically not used to seal anything under the hood, but can be found in various places around the body of the vehicle due to its waterproof qualities. This silicone is the automotive version of what most fish tank corners are lined with, and has similar uses in households. The automotive version is designed to be more resistant to changing weather conditions, slight twisting, and moderate heat. A more runny version of the clear formula is commonly used to seal set-in windows, like most modern windshields.
Other Gasket-Forming Materials
Although RTV silicone gaskets are the most common gasket forming material, there are other options which may be better suited for certain situations and applications.
Anaerobic Gasket Maker
Anaerobic (meaning "without air") gasket makers start as a clear colored gel which can set up and cure in the absence of air. These are typically used for on-the-spot repairs and to repair broken gaskets.