Navtex broadcasts are all in English on SSB on 518 kHz. What's the best you can do to receive a message and what do you need for receiving it. The varying tones correspond to varying shades of gray that the satellite sees as it scans the earth. The earth is scanned every half-hour where the raw data is transmitted to a receiver station requiring a 60' dish with sophisticated computing equipment. The data is reformatted in real time with political boundaries added and transmitted to the satellite where it is retransmitted back to earth at 1691 MHz.
Radiation passes here without severe attenuation by the intervening atmosphere.
Weather satellites do not have sensors that cover the visual spectrum to produce true colour images. Visible satellite images provide information about the observed cloud cover. Areas of white indicate clouds while shades of gray indicate clear skies, this is because thicker clouds have a higher reflectivity and appear brighter on a visible image than thinner clouds IR imaging is needed, as weather watching is essential day and night. Colour enhanced infrared satellite images are measurements of temperature thus only the differences in temperatures are visible. In an IR image darker is warmer and lighter is colder.