Unlike solutions posted elsewhere, you do not need to download an icon from the Internet. This solution creates an appropriately sized monochrome icon from the existing Spotify indicator icon already installed on your system.
Here is a picture of the new monochrome Spotify indicator icon.
Create Monochrome Icon
Make a backup of the original resources.zip
file, which contains the Spotify indicator icon.
sudo cp /opt/spotify/spotify-client/Data/resources.zip /opt/spotify/spotify-client/Data/resources.zip.original
Extract the Spotify indicator icon to modify it.
unzip /opt/spotify/spotify-client/Data/resources.zip _linux/spotify-linux-512.png
Install and check your version of ImagicMagick.
We will use ImagicMagick to create the monochrome icon. With ImagicMagick version 6.8.4.x, there was a major change in the way ImagicMagick treats grayscale images.
sudo apt-get install imagemagick
convert --version | grep Version
Create the new monochrome Spotify icon.
For ImagicMagick version < 6.8.4.x, do the following. If you are using Ubuntu 14.10, you will have ImagicMagick version 6.7.7-10.
convert _linux/spotify-linux-512.png -resize 448x448 -background transparent -extent 512x512-32-32 -colorspace gray -sigmoidal-contrast 10,19% _linux/spotify-linux-512.png
For ImagicMagick version => 6.8.4.x, do the following. If you are using Ubuntu 15.04, you will have ImagicMagick version 6.8.9-9 or higher.
convert _linux/spotify-linux-512.png -resize 448x448 -background transparent -extent 512x512-32-32 -colorspace gray -sigmoidal-contrast 10,50% _linux/spotify-linux-512.png
Update Spotify with the new monochrome icon.
sudo zip /opt/spotify/spotify-client/Data/resources.zip _linux/spotify-linux-512.png
If Spotify is already running, exit Sotify and restart it for the new icon to become available.
Remove working files.
rm -rf _linux
Undo Changes
To revert to the original color Spotify indicator icon, execute the following.
sudo cp /opt/spotify/spotify-client/Data/resources.zip.original /opt/spotify/spotify-client/Data/resources.zip
Software Updates
Whenever you update Spotify, the custom icon will be overwritten, so simply perform steps 1 thru 6 to recreate the monochrome indicator icon.