If your 156 is bright red or some other non-metallic colour, you will find it easier to touch in scratches than if it's metallic. The best result comes from very fine wet sanding later (1500-2000 grit) and polishing. It is quite a time-consuming process. Best to call in one of the paint touch-up SMART repairers if you haven't done this sort of thing before, especially if the car is otherwise pretty mint.
I always buy 'proper' paint from a paint shop, mixed to the recipe, in a 250mL tin (enough for painting a bumper, spoiler, mirror, etc. should the need later arise). I just shake the tin (it's best to stir, especially if metallic), remove the lid and cover the tin, and use the point of a cable tie to dab the paint into the chip/scratch, taking paint from the lid of the tin. Replace lid and shake, and it sits there for another six months between uses... worst thing is when you have a whole shelf of such tins and snails eat the labels or the labels fall off... now, was this Honda Shoreline Mist or was this Alfa wheel silver...
For a metallic colour, you'll be buying the (matt) basecoat, no hardener required. I find it still polishes up to an adequate shine anyway. For repainting parts properly, you'll need the clearcoat (seems to be called 'lacquer' in England) as well, but I don't think you need that for touching-in.
Stone chips are much easier to touch-in than scratches, in my opinion, and so that is the best thing to try your hand at first. Plus, scratches may polish out with the right compound, especially if you can't feel the scratch with your fingernail. You wouldn't want to touch-in where a polish would do the job instead.
EDIT: Sorry, but those 'paint pens' look like a total sham to me!
What a ripoff! All it probably does is to 'gloss' the edges of a clearcoat scratch that appears white, which you could do by polishing anyway.
For that price you could easily get the correct paint mixed at an automotive paint shop and as I said, have it ready in case a bigger repair is ever required