Hello fellow Astartes, as I've been asked about this a few times, I figured out making a video demonstration/explanation would be more useful to everyone. I don't have much experience in making tutorials, so if I left out anything, feel free to ask.
As someone who doesn't own an airbrush yet, and who doesn't want to spend ages painting a few models, I figured out how to do wet blending quick enough to paint the gradients on a marine's armour in just 5min. Doesn't need setup like an airbrush, just a flat brush and geedub colours. Obviously not Golden Demon standard, but quick enough to paint entire companies without dying of old age.
Also, the same works for more than 2 colours. My multi melta barrels' heat stains for example are a blend of (1/3 each) leadbelcher, genestealer purple and maccrage blue.