or use signs to label your chests....

https://steamcommunity.com/sharedfiles/filedetails/?id=2643985634

not sure if youre into mods but https://valheim.thunderstore.io/package/ComfyMods/ColorfulPieces/ allows you to color the chests but i recommend you install a config manager so you can change the colors if not you will have too use the config editor built into thunderstore here is the link for thunderstore as well in case you are interested https://www.overwolf.com/app/Thunderstore-Thunderstore_Mod_Manager

This is such a good idea,that I am confident the devs will never do it

Getting a bit side tracked here, but... being able to paint objects, not just chests, could be fun. But I also think that signs are a better choice, colorblind friendly and everything.

you can make a sign with 2 wood and 1 coal I believe. I use it to make small houses for chests and label that way. hth.

While colors would be nice, there is a sign you can make to put symbols or words next to or onto things.
I want colors too, but until then...
Oh, and you can do colored shields for one of the types as a decoration, though its not worth using.