Hypervisors allow sharing of computing resources between applications—possibly of various levels of criticality—that makes them increasingly relevant for modern embedded systems. In this context, memory isolation properties (including low-level cache isolation) are essential to guarantee. This paper presents a case study on formal verification of the cache coloring mechanism implemented in the Bao hypervisor. It proposes an original technique for coloring memory pages and assigning to each virtual machine only pages of certain colors, aimed to provide strong isolation guarantees. The implementation presents several challenges for formal verification, such as bit-level operations, complex arithmetic operations, multiple levels of nested loops, and linked lists. We identify two subtle bugs in the existing implementation breaking the expected guarantees, and propose bug fixes. We provide formal specification for the key functions of the mechanism and verify their (fixed) version in the Frama-C verification platform with a few lemmas proved in the Coq proof assistant. We present our specification choices, verification approach and obtained results. Finally, we outline possible optimizations of the current implementation.