JenG,
PReS v3.1.10 and v2000.102 are quite dated. Version 6.1.6 is the latest and has many changes regarding charts and color handling. (For a copy of this version, please contact your sales representative.)

The output language is one area that needs to support COLOR output. The second is the use of an appropriate COLOR instruction. I do not know what/where/how in those versions to accomplish this. You will need to review the documentation. I could be as simple as adding MODEL "COLOR" to your script.