You have recipes that predict 3.5 SRM but as a home brewer you have never produced a beer this light. It is almost impossible. That said you don't really care what the numbers are, you would like the beer to look darker.
As it turns out all beer color is essentially the same and comes from a fictitious 'beer color factor' whose concentration determines the color of the beer (Kriek etc aside). To increase color all you need to do is add more beer color factor. As it is the same color (has the same spectral absorption characteristics) as the beer color factor already in your beer adding more of it will not change the spectral color characteristics of your beer. The perceived color will change, obviously, as the concentration of beer color factor increases. As BCF concentration(SRM) goes up the beer becomes darker and its hue describes a clockwise loop, starting at the origin Lab space a*-b* coordinates ranging fom white (SRM 0 - no BCF) through green-yellow, yellow, orange, amber, red, deep red, deeper red and finally back to the origin. The SRM number is proportional to the concentration of beer color factor so all you need to do to get more SRM is add more beer color factor. Several methods for doing this have been suggested e.g. add some malt which has more factor, enhance the existing concentration by longer boil (heat produces it and boiling away water concentrates it).
One method, and probably the simplest, is to buy it in a bottle. The product is called Sinamar and Williams carries it. Just add enough Sinamar to get the color you want. Caramel also has it so you can melt/burn some sugar on the stove to produce a caramel with color deep as you want it and then add as much as you need. This, of course, adds caramel flavor. Sinamar does not.