I stumbled on a neat bit of kitchen science this morning, which you can see illustrated here:
Both the murky grey puddle on the left and the cerise splash on the right are essentially the same liquid; a harvest of berries from my myrtle bush (Myrtus communis) boiled for a while with some water and a healthy dose of sugar. Okay, maybe that should be an unhealthy dose of sugar but that’s what you need to make a syrup.
What is the difference? The one that looks like you might want to pour it on your icecream is a sample taken just a few seconds after adding lemon juice to the pan. I think it is doing something to the pectin from the fruit, allowing it to clump together which would allow the mix to set if I boiled off enough water. However, in this case, I’m not after a set but there is probably something about how the changed structure of the liquid reflects the light that leads to the much improved colour.
So, for fruit-based sauces, a lemon or other acid-contributing ingredient seems a key element to remember.