Try to pick the elderflowers when they are big and blousy and have had some good sun on them. Don't pick first thing in the morning as they are just not as good for making cordial and if they have had some sun on them it makes for a more fragrant cordial.
If you want to make the cordial pink then use red rose petals as they impart their colour more effectively. I have tried it with pink roses and you just don't get the blush colour - it is much lighter and when diluted disappears. If you don't want to use them just leave them out.
The season for Elderflowers is short and it freezes really well so I make enough to last me the year. I collect 500ml or 1 litre plastic bottles over the year and then freeze the cordial in those plastic bottles and pull out when needed.
I do use white sugar here as golden granulated makes the cordial a golden honey colour but I prefer a clearer cordial.