Cochineal is very sensitive to changes in ph, so if your colour came out purple, it usually means that your water is a little bit alkaline. It's one of the fun parts of cochineal - that you can shift the colours all the way from pink to red to orange to purple, but also means that (especially as a first time dyer) you don't know what you're going to get.
You can shift the colour a little bit after it's dyed by putting it in a bucket of water that has a couple of glugs of vinegar in it - you should see it shift a bit more towards pink.
But for dramatic shifts in colour, you need to add the acid or alkaline at the time of making the extraction - once there is a base colour on the fibre, it's harder to change completely.
If you are aiming for pink and red shades rather than purple, put a glug of white vinegar or the juice from a lemon or lime in with the ground insects the first time you go to make your extraction.
Iron rich water will also have an effect on cochineal, shifting it to purple. If you are on well water and know that your water has iron (even in trace amounts), you may want to use spring or rainwater for your dyeing.