The default icon is green for a new message, orange works much better to catch the eye.
Hit alt+f2 and type 'gksu nautilus'
Naviagate to the icon theme directory you're using in
(This will be either ubuntu-mono-light or ubuntu-mono-dark.)
Enter the status/22/ directory
Choose your preferred coloured-icon from this folder
IMP0RTANT: RENAME it to just 'indicator-messages-new.svg' before moving.
Move it and enjoy the "non-green" message icon.