Hi! I understand your problem and I may be able to help you eventually. 😄
My artstyle also has a quite retro/ old school tone to it. Even though I'm not sure if it's the tone your looking for. But I think I'm able to give you some tips.
You can give any of your artwork a nice brown, gold or even yellow tone in which you use specific pencils or markers (I specifically like using copic markers)
I usually do the lineart first and then do the coloration with copic markers. Make sure that the coloration is not too dominant and that it's pale enough so you can add the brown/yellow(...) tones later.
With the help of the brown/ yellow tones you can add shadow or maybe even little spots to make the drawing look kinda old.
My personal favourites are brown gold markers. I like using my brown fineliner for the lineart. I don't know if you use lineart the but this method definitely works. It helps giving my drawings a nice retro atmosphere. Of course you can add some extra color if you want to but don't exaggerate. After all we still want to keep that nice warm retro atmosphere.
On the other hand you can use brown transparent sheets of paper that you can glue over your drawing. Although I don't use this method a lot.
Also what helps is using water colour. For instance your drawing a building and you add coloration (pale colors) and afterwards add a whole layer of water color. Paint all over the intire drawing. Note that the water color can't be too dominant either. Otherwise we won't be able to see the color of the building anymore. The water color is supposed to be a contrast (so to say). So it'll still look natural.
I hope I could give you a bit of a advice.
If not that's fine too. XD
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1. Picture are the fineliner I use for my drawings
2-3. Picture is a drawing I did with copic markers, colored pencils and my fineliners
4. Picture is another example 🙂