Just finished this beautiful Viper kit from moebius, I real pain to mask all the red stripes but well worth it. I might add scuffing marks but will let it rest for a few days first and start with a fresh mind.
I first sprayed the model using Duplicolor grey filler primer which I use on all my models, Base color was airbrushed using Testors Model Master light grey, before airbrushing the base color I masked the grey stripes on the engine cowls, the nose tip and the 2 panels on fuselage so they are basically the duplicolor primer color showing threw, for the red/orange stripes I used Humbrol 132.
its called a wash , very diluted dark or black paint, you need to clear gloss your model first to protect the finish to wipe off excees, you can see many videos how its done on youtube