Downloading the update couldn't be any simpler – if you've already downloaded the Nintendo Letterbox, all you have to do is head over to the Nintendo eShop, where you'll be prompted there are updates available for our software. Clicking 'View List' brought up a list of games with updates, and then clicking the 'Update' box on the Nintendo Letter Box section set it downloading, either straight away with the 'Download Now' option, or 'Sleep Mode Download', which downloads in the background when your system is closed up doing nothing.
If you aren't prompted to download the update upon launching the Nintendo eShop, you may need to find it yourself – in which case, the easiest way to do it is to search for it in the search box at the top of the front page of the eShop. Even searching just for 'letter' will return the application (third one along), and clicking on the option for 'Nintendo Letter Box' will take you to it's official page, where you can chose to download it if you haven't already, or update it if you missed out on the automatic prompt.
Once you've updated everything, you should now be able to choose from red, blue, green, purple and orange coloured inks. To change the colour you're writing in, simply press left/right on the +Control Pad to scroll through the selection of colours to find the one you want. Unfortunately, you can't mix and match colours, so if you choose purple, your entire message has to be drawn in purple – so you can't create any fancy coloured drawings yet. Boooo. Maybe in next year's update.
There's not really a lot else to say, except happy scribbling with your new found colours. In the meantime, we're going to keep our fingers crossed for a rainbow ink similar to that found on the DSi PictoChat applications.