|Left: Ultramarines Blue. Right: Altdorf Guard Blue|
Ultramarines Blue is on the left and Altdorf Guard Blue is on the right. At the top of the hatch I painted a small streak of Altdorf into the Ultramarines and at the bottom I painted a small streak of Ultramarines into the Altdorf.
The Altdorf appears to be slightly darker, but It's similar enough that I doubt I could easily tell the difference between a model painted with Ultramarines Blue and a model painted with Altdorf Guard Blue, especially if a generous wash were used. The most noticeable difference is that Altdorf is most definitely a "layer" paint. The Ultramarines Blue covered the gray plastic with only two or three coats. The Altdorf Guard Blue, on the other hand, required four or five coats before it completely covered the gray. Altdorf should probably be painted over a blue base coat for best results.
For my Ultramarines I've primarily used Mordian Blue for the power armor and reserved Ultramarines Blue for highlighting. However, I use Ultramarines Blue on my vehicles so they contrast slightly with the Marines. Given how much Altdorf Guard Blue was needed to cover a color other than blue, I'll definitely be priming my vehicles with Army Painter's Ultramarine Blue spray rather than a black or gray undercoat.