In anticipation of future multifrequency observations of black hole with the Next Generation Event Horizon Telescope, we construct spectral images of a thin accretion disk around a spherically symmetric black hole immersed in cold, nonmagnetized, pressureless plasma. The radiation from the disk is assumed to be thermal, and the surrounding plasma is entrained by its rotation. We use the general relativistic transport equation for the radiation in the plasma, accounting for both dispersion and plasma motion but neglecting absorption. Shadow images and intensity maps are computed across the full spectrum for an inverse power-law plasma density profile. The results show a strong dependence of the observed images on the radiation frequency, which looks promising for the possibility of extracting new information in future observations of the Next Generation Event Horizon Telescope.

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• Classical black holes
• General relativity
• Gravitational lenses