Nick Larsson (aka frals) has posted an article on adding a small (120x120px) logo to your N9 lock screen.
The N9 has a PenTile AMOLED screen, but is configured to avoid the colour fringing problems that affected the Android-based Nexus One. However, when the lock screen is displayed, certain bit patterns produce colours:
John Hutchison's Generating false colour images on the Nexus One using only grayscale pixels contains source code and examples.
The above photo was created by taking a 120x120px cut from the example rainbow image and setting it as the logo:
The same section, viewed on a Nexus One looks like:
As you can see, the colour mapping isn't the same (meaning new reference images need to be generated). However, you'll see it is possible to have colour logos on the lockscreen. Hopefully someone will take it forward, perhaps Nick can update his tool to do colour mapping; or someone can post the reference images so that the Java source code from Luke Hutchison can work.