Yvonne, I used to keep frogs and toads for many years and reared them from egg stage onwards.
What aspect is your garden pond? (north, south, east or west facing) Unless it is very deep, it is possible that it will get very warm to hot if it is in direct sunlight and facing south to west. This will kill the frogs very easily.
The pond should ideally be partially to heavily shaded by plants (at least 50% shade) for the hottest part of the day in midsummer. There should also be shade areas around the pond, such as low-growing plants with broad leaves, flower pots (clay/terracotta) or similar.
The discoloration is a normal decay indicator.
The survival rate of froglets is very low, not surprisingly, given their tiny size, which makes them a very appealing snack, but you can give them a good start!
