You can do this using the DSPATR keyword and a "program to system field"
It is fairly messy to do in 2E (it requires user source or RPG program, scan & replace CL, etc), but once set up it does work OK.
https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_73/rzakc/rzakcmstdfdspat.htm
First, create a 1 character field in the model to use as the program-to-system field. You will need to be able to set this to an appropriate hexadecimal value (as in the above link) for the relevant colour, for example in an RPG program.
You then add the program to system field to the same screen as the field you want to make various colours, then use a scan and replace program to change the DSPATR on the multi-colour field to use this program-to-system value. This can be attached to the DDS to run automatically after the screen has been generated.
I can probably find some code examples if this helps.
- Bob