The acid colorless organic compound smelling like rancid butter is butyric acid (also known as butanoic acid).

Here's why:

* Colorless: Butyric acid is a clear, colorless liquid.

* Organic compound: It is a carboxylic acid, a type of organic compound.

* Smells like rancid butter: Butyric acid is responsible for the unpleasant, rancid odor of butter that has gone bad. It is also found in other dairy products, sweat, and vomit.

Let me know if you have any other questions!