My friend showed me how to do this once and I noticed there were a lack of good tutorials showing how to change a folder colour so I decided to make a set of simple, easy to follow instructions!
Step 1: Right Click to Create a New Folder
Step 2: Right Click of Folder and Select : "Get Info"
Step 3: Highlight the Folder Icon on the Top Left and Copy (⌘c)
Alternatively, once the icon is highlighted you can press Edit then copy.
Step 4: Open the Application Preview
I opened it by searching for it in the spotlight search but there are many other ways to access it!
Step 5: Press ⌘n
Alternatively, you can press file then new from clipboard.
Step 6: Select Adjust Colour From the Tools Menu
Step 7: Play Around With the Tint, Shadow Highlights Etc.
Until you have reached your desired colour!
Step 8: Press on the Folder Icon As Shown and Copy (⌘c)
Or you can press edit, then copy.
Step 9: Quit Preview (⌘q)
or Preview, then Quit Preview
Step 10: You Do Not Need to Save the File, Select Delete
Unless you really want to!
Step 11: Return to the "Get Info" Section of Your Folder and Select the Folder Icon.
Step 12: Paste the New Folder Icon by Typing ⌘v
or pressing edit, then paste.