A 28-year-old woman from Delhi presents with acute onset fever, rigors, and a petechial rash on her trunk and lower extremities. Lumbar puncture is performed; CSF is turbid with elevated protein (180 mg/dL) and low glucose (20 mg/dL). Gram stain of CSF shows Gram-negative diplococci with kidney-bean morphology. The organism is oxidase-positive and ferments glucose and maltose. Which staining characteristic of this organism makes it difficult to visualize in routine clinical specimens and necessitates the use of special stains or immunological methods?