Why Analog SSTV transmits an image line by line as an audio-frequency FM tone, where the instantaneous frequency represents brightness (roughly 1500 Hz black to 2300 Hz white). There is no room for a separate color subcarrier in a 3 kHz SSB channel, so color modes like Scottie and Martin send each scan line three times in sequence, once for each color component (red, green, blue, or luminance plus two color-difference signals). The receiving software reassembles the sequential color lines into one full-color line.
Watch out The color burst idea comes from analog fast-scan NTSC television, which uses a 3.58 MHz color subcarrier with a burst on the back porch of each line; SSTV has no such subcarrier.
SSTV is slow because it sends each line three times: red, then green, then blue, one color at a time.