The fans come on so the switch is fine. The compressor relay is fed from the same place as the fan but in series with the pressure switch (to earth), giving a 99% certainty that you are low on gas!
most main dealers should do it.
if the switch is dim when turned on i would be looking at a seized compressor so make sure they have a look when you get it done.
The switch may be dim just because the lamp inside it is knackered.
Hint: the green lamp is the same as fitted inside the alarm cancel button - much more easy to find at scrap yards