you can use the value color property of the column and define the same color for multiple entries:
... alternatively you can define a new column which can be used for the COLOR - and continue to use your original column as Overlay.
Here is an example with Measles data from the STIPS course. Separation by Region is just applied to clearly see the different color groups.
Open( "$SAMPLE_STIPS/Measles.jmp" );
Graph Builder(
Size( 1061, 493 ),
Summary Statistic( "Median" ),
Graph Spacing( 4 ),
Variables(
X( :Year ),
Y( :Cases ),
Group X( :Region ),
Overlay( :State Abbr. ),
Color( :Region )
),
Elements( Line( X, Y, Legend( 5 ) ) )
);