If you don't want a legend then row legend is possibly the not best route. Interactively in JMP you can colour using the Color Or Mark By Column option on the rows menu. In JSL this is a message that is sent to the data table e.g. dt << Color Or Mark By Column( :Group). Check the scripting index for additional arguments - colour themes etc
You can also accomplish this without scripting from your data table by going to JMP -> Rows -> Color or Mark by Column, and then choose the column by which you would like to color your data. This will color by all rows, not just rows in the "by" group, and will not automatically add a legend.