findericon
So I symlinked my Downloads folder from my boot drive to an external HDD I always keep connected to my mac. I deleted the original downloads folder from my home folder and replaced it with a symlink to a new folder on my HDD.
This works perfectly, except that in Finder, the sidebar icon is now a folder:
When it should look like this:
Operating system is High Sierra 10.13.6
Goto your Home Folder > Library > Preferences and delete the files:
com.apple.finder.plist
com.apple.sidebarlists.plist
If you can't find the files use this method: from the Finder menu bar, select Go > Go to Folder and copy the following line into the text box that opens:
~/Library/Preferences/com.apple.finder.plist
Then, restart, or log out and in again.
Select Go > Go to Folder and copy the following line into the text box that opens:
/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/
Here is where you will find all the sidebar icons -
Now you will have to manually change the icons for the respective folders by using Get Info on the folder that is in your sidebar and by dragging and dropping the icns file onto on the Inspector panel.
try this in Terminal
mkdir ${TMPDIR}/com.apple.IconServices
This recreates the com.apple.IconServices directory, which apparently gets itself deleted from time to time.
Best Answer
Even though what you need is very little, there seems to be no easy workaround except for the SIMBL plugin:
How can I change Finder's sidebar icons in OS X El Capitan?
As an experiment I uploaded my downloads folder to google drive. You can try downloading it and see if it works:
https://drive.google.com/drive/folders/1U3dY4N_ZDLtnq93yoKVYVi4F5WqBaa8i?usp=sharing