The RasMol charge colour scheme colour codes each atom according to the charge value stored in the input file (or beta factor field of PDB files). High values are coloured in blue (positive) and lower values coloured in red (negative). Rather than use a fixed scale this scheme determines the maximum and minimum values of the charge/temperature field and interpolates from red to blue appropriately. Hence, green cannot be assumed to be `no net charge' charge.
The difference between the charge and temperature colour schemes is that increasing temperature values proceed from blue to red, whereas increasing charge valuse go from red to blue.
If the charge/temperature field stores reasonable values it is possible to use the RasMol colour dots potential command to colour code a dot surface (generated by the dots command) by electrostatic potential.