The plant was probably mislabelled (it happens, and sounds likely as the pink edging should have been present when you bought it). I would contact the supplier to arrange a replacement or refund.

It should colour up in partial shade or full sun, although my experience has been that the varieties of Phormium with red leaves actually develop a better colour in partial shade - they can look bleached in full sun.

I'm wondering if it was showing much pink when you first bought it - Phormiums are incredibly variable, and its always best to see before you buy and just pick one you like the colours of - many are mislabelled or misnamed, and there are now hundreds of varieties. If it was showing good colour when you bought it, then change its position - it may be its not responding well to the light levels its got currently in your garden. If it had very little pink and doesn't look much different from when you got it, frankly, I'd look for a replacement with better coloration. Its still buyer beware though - some are hardier than others; 'Joker' is listed as half hardy with some sellers.

There is another, more reliable, hardy red leaved variety, often sold as 'Sundowner', but its not pink by any stretch of the imagination; there are others sold simply as 'red phormium' - I'm a frequent user of internet plant suppliers, but Phormium is something I prefer to seek out and look at before buying because of their variability and the unreliability of the names.