I don't think a colorset manager default with Blockland, there used to be with RTB but you can install colorsets manually.
Go into your Blockland directory, then config, then server, and then there should be a .txt file named colorSet.txt in it. That's the current (default) colorset that comes with Blockland.
Open up the .zip file that you downloaded from the above website and inside it there is a colorSet.txt.
Replace the one in the zip file with the one in the server folder then restart Blockland/your server and you're good to go.