At the top of these orange beds the fossils have been washed around and damaged by wave action, indicating a very shallow environment of deposition.
A close look will show that the fossils here include very few actual shells. This is because many sea shells are made of aragonite, a form of calcium carbonate that differs in its structure from the other common alternative which is calcite. Aragonite tends to dissolve relatively easily during the rock forming process, and to re-precipitate as calcite in the matrix of the sediment. This makes these rocks very hard, but with many gaps where shells have disappeared, leaving only the internal casts.
In this photo you can see some trace fossils made by some sea animals burrowing into the sediment about two million years ago.
So why do the rocks show this change from the grey muds, deposited in relatively deep water, to progressively shallower sandstone and limestone? Either the land was going up or the sea level was going down, or perhaps both were happening at the same time.
The rocks around Hawkes Bay and other parts of New Zealand show clearly that the main cause was sea level change, which in turn was due to global ice age cycles which themselves were driven by changes in the earth's orbit around the sun (called Milankovitch Cycles). So if you ever go to Waipatiki for a holiday, you may like to look for some fossils and consider the relationship between Astronomy and the colours of the cliff.