Don't get pen..its just Blanco and is useless. Get ur car matching color from any auto paint shop with hardener in it. Keep it in a sealed dabbi. Don't use plastic dabbi as color will loose its liquification a lot earlier. Use any small metal dabbi..if u can find.
Now clean the spots with turtle green compound as it's silicon free. If you don't have compound, clean with mobile screen cleaning solution. Apply qalam/touchup paint with cotton bud as a bit thicker and leave it. Dont wash car for 2-3 days and after a week , rub with 3000 grit paper (3m is best and can be bought from auto hub) using water very gently. Once touchup paint feels smooth to touch, apply compound ans after that apply any good wax. After a couple of month, it will be harder for you to find touchups. When ever u can polish the car with machine.. touchups will further smooth out and blend in.
These steps may seem hassle but actually its a simple process and these touchups will not ne visible easily and this is the professional way to do it. Plus, due to hardener in it, this is permanent.
Total cost will be around 250 for paint and normal 3000 grit paper is worth 50-100 pkr. Once done eith touchup, throw the paint. If u cant find metal dabbi then use the paint on same day u buy it.
