• Red -  high frequencies (HF) are unavailable (>22 MHz)
• Yellow -  lowest usable frequency (LUF) and maximum usable frequency (MUF) range is 15-22 MHz
• Green -  LUF-MUF range is 8-15 MHz
• Blue - LUF-MUF range is 3-8 MHz

If a communication link is not working at these frequencies, there is a higher probability of malfunctioning equipment and/or jamming as a cause for outage.