Either find a touchup paint that closely matches (often bike shops have tons of surplus or can order one that matches), or clear coat it. Or use clear nail polish.
Or don't worry about it ;)
As long as you're not storing the bike outside in the elements, you will reach the fatigue life of a weld somewhere on the frame before scratches will rust sufficiently to compromise its structural integrity.
And before anybody says it- yes, I know steel can be repaired. Once again, as long as the bike is being stored out of the elements you'll be able to repair welds many times over before the frame will rust through.