Nokia has said they will only be offering Black, White and Cyan Nokia Lumia 900 handsets.
Interestingly however Nokia has a facebook page where one can try out the handsets, and also change the colours, and this does in fact feature a Magenta option.
The other handsets in the Lumia range are also available on the page, and interestingly the colours vary by handset, for example orange is only available for the Lumia 710, suggesting at least some customization is going on.
There has been a rumour the Magenta device may come out on Motherâ€™s Day, which would be a great bit of marketing on Nokiaâ€™s side.
The page can be seen here.
Thanks Michael for the tip.