exact pins the design to one length and lists it ahead of the buckets.
#00000000 = transparent.
A slot is a named colour. An element set to primary looks the
colour up here, so changing it once recolours everything using it —
that is how a logo gets a red and a blue version without rebuilding it.
You are not limited to these: any element can take a fixed colour instead,
with custom hex in its own Color dropdown. Use a slot when
the colour is part of the palette, a fixed hex when it must never change.