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.