Hi Dina
As you know I have this problem too! I have a canon powershot camera, not quite an SLR but not too bad and I use a lightbox with bright white lights (though believe me I’m not sure if I’m doing it right at all!!! :) and I still can’t photograph purple very well! So I have just recently purchased Adobe Lightroom and (hopefully the picture is attached) have been playing around with hues! As you can see by the picture you can change the hue of the purple quite a lot, the image on the left is the original one and the image on the right I have adjusted the purple hue quite a bit! Et voila! A great way to adjust purpleness (is that a word?) in your photos. The only problem I can see is however, you would have to edit your photos when you still have the cake with you to get the shade as close to what you see in real life as possible, as I made this cake a couple of weeks ago now and I can’t remember what it looked like in real life! Anyway, Adobe lightroom have a free 30 day trial you can download and give a whirl, there is also lots of other cool functions on there too. Hope this helps a little bit! :)
