good luck, to my knowledge my entire car is WRB, yet, in direct sunlight, my rear bumper looks different than the rest of the car. i cant tell if its the top coat or the color is off, but it is slightly off. this is a 2018 100% stock car. i dont really care as i doubt the average person would even notice and i have no plans to ever showcase my car in any way but it is there nonetheless.
so the point is that there is a spectrum of WRB even from the factory. your paint pen will likely not be perfect ither...
that said i ordered some gun metal touch up color for my g37 to do a few touch-ups before returning it at the end of my lease and it matched very closely. so to throw another hat into the ring, you could try
https://www.automotivetouchup.com that is probably where ill get paint for my wrx when the inevitable rocks begin assaulting me.
one final thing, i think having a brush and a bottle of paint is better than the felt pen things... im sure some people swear by the pens and hate brushes but i personally much prefer a brush for painting.