This paper introduces the language-independent concept of ``thread usage policy.'' Many multi-threaded software systems contain policies that regulate associations among threads, executable code, and potentially shared state. A system, for example, may constrain which threads are permitted to execute particular code segments, usually as a means to constrain those threads from accessing or writing particular elements of state. These policies ensure properties such as state confinement or reader/writer constraints, often without recourse to locking or transaction discipline.

We introduce a thread colors model as a way to express design intent concerning the relationships between threads, executable code, and shared state. By expressing the model as annotations in code, it is possible to formally link the model with source code and to analyze the consistency of model and code in a composable manner. By using annotations as cut-points, APIs can be annotated and compliance with library threading policies can be evaluated. This is illustrated using case study examples from published code that show how thread coloring models can assist in assuring policy compliance and in identifying concurrency errors.