Altona is one of the most often recommended mophead hydrangeas, known for strong colouring and for turning deep crimson in the autumn. At The Cross we have found it reluctant to produce many flowers any more blue than muddy mauve, and we have never seen the autumn crimson.
The original plant, growing near Europa and Hamburg, does produce a few fairly attractive blue heads (see examples below), but the colours shown above are much more typical. The striking difference from the pot-grown plant is that none of the florets have shown any tendency to serrated edges.