purple is a favourite colour of mine but it only comes into it's own in clear water, as does blue.
it's all to do with light availability, water clarity and the different response of colours to light throughout the colour spectrum.
red looks grey the deeper it is and is said to happen at around 15ft, whereas the blue end of the spectrum, including purple, is still visible at depth... if the water is clear.
Water clarity plays a big part here though. If there is a lot of suspended particles, ie murky water, then all basic colours disappear to some extent and that's when fluoro colours, and bright colours like white, yellow, chartreuse etc, are still visible. A basic or prime colour lure in deep, murky water will become pretty much invisible unless it's inches away from a fish. In waters that are normally coloured it isn't so much of a problem as the pike have learnt to use vibration/water pressure changes as their primary hunting method but in a water that's normally clear that turns dirty it's often a waste of time.
one exception i've found though... glow in the dark lures seem to work best in clear water, and in daylight, for me at least!? Work that one out!