The standby temperature is set by the R parameter in the G10 command. However, a heater needs to be turned on before the standby temperature is applied. Here is a possible sequence if you are starting with the tool heater turned off:
G10 P0 R100 ; set standby temperature of tool 0 to 100C
T0 P0 ; make tool 0 active without running tool change files
T-1 P0 ; put tool 0 on standby without running tool change files
If what you want to do is to set the standby temperatures for a two colour print, just use the appropriate G10 Rxxx commands in your slicer start code.