It's easy for the average person to get concerned and confused over what is leaking from an engine. There are many fluids which are found in the engine as well as in the engine bay. Maybe of these can be identified by smell and/or color.
Sometimes the known color of a fluid will not help much in identifying a leak. For example, no matter what color antifreeze you have (they come in many different colors) it will almost always look like a simple water spill on concrete if the leak is slow enough for it to absorb in. Many fluids that are past their working life turn black, so no matter what color they started out as, it will become difficult to distinguish one worn out (typically petroleum-based) fluid from another.
The Ultimate Car Leak Color Guide by Cars Simplified
We don't recommend smelling any vehicle fluids (for health and lawyer reasons), but if you happen to detect a smell, here are some descriptions:
Antifreeze
A strong, sweet smell. Because of this, animals are likely to attempt drinking it (do not let them, it is quite toxic). Its color differs from brand to brand, and sometimes changes color with use. Most common antifreeze ranges from green to yellow, but propolyne glycol is orange, and OAT/HOAT coolants come in nearly every basic color. RV/Marine antifreeze is typically red or pink.
Automatic Transmission Fluid
This fluid does not have a notable smell, but is often a pinkish red when new, and darkens from red to brown with use.
Brake Fluid
This fluid does not have a strong smell, and looks like blackened water after use. Some performance brake fluid has a light blue coloring to it, but this fluid type isn't street legal in the United States.
Diesel Fuel
Similar to gasoline in both scent and color, but slightly darker yellow/orange. Diesel in poor condition may contain algae.
Gasoline
If you've ever been to a gas station, you've probably recognized the strong smell of gasoline. It is quite trnasparent with a slight yellow tint to it.
Motor Oil
Motor oil has a moderate smell, but it changes depending on the type, brand, and how long it has been used. We don't recommend this, but if you need to know what your oil currently smells like, you can take out the dipstick and smell the tip (where the oil should be). It also often has a colorful look to it when on pavement, depending on how new it is (aged oil often loses its color effect).
Power Steering Fluid
Water-like with a hint of amber, and mostly scent-free when fresh, with the potential for a bit of burned smell when used and potentially a strong burned smell when in bad condition. This is unless the system calls for ATF, which is red, and all of the ATF features are the same as described there.
Racing Fuel
Racing fuel tends to have a slightly different smell than gasoline, but if you use it, you will be familiar with the smell. Racing fuel typically evaporates quickly, making the vapors an explosion risk beyond the already present risk with gasoline.