Well spotted ! 
This is a very simple trick that I like to use when I have to work fast / when I’m short on time.
It’s just a 50% grey layer, with the blend mode set to Overlay. Then I use a noise filter with gaussian blur. Also, I typically throw in another layer of the merged image, blur it a little, with the blend mode set to Lighten and a very low opacity. (To top it off, there’s also a little bit of chromatic aberration)
It’s cheap but I like it.
And like you said, it really brings everything together !
Thank you ! As a matter of fact, I actually did a little bit of comics. The project is on hold but you can read a few pages here : Les aventures de Cybergirl. It’s all in black and white though…
@Sutansan, @hokutonokatani, @POSEIdooN : Thank you all for your comments ! It really motivates me ! Also, I haven’t had the time to comment on all your threads but I’ll do so when I can ! There are so many great works everywhere ! :bounce: