If soil acidity alone were the determining factor in color, one could easily run some controlled experiments: grow the same croton in an acid soil and then a basic soil, keep all other conditions equal (grow them next to each other). Use a sample of six crotons to give this a modest size sample. Grow a set in FL and another in HA. Keep decent records and report back in a year or so. Any volunteers?
If I recall correctly, one can easily change blue to pink hydrangeas and back fairly easily. But with hydrangeas we're looking at the flowers; with crotons it's the leaves.
Back to crotons: I think most of us in FL try and keep the croton planting beds on the acidic size. I've been using anywhere from 5-10 cu.yds of recycled mulch every year to save water and improve the 'soil' here which is mainly sand. Coffee grounds also go in the garden. I think the 'soils' further south in FL range from marl to the muck in the Everglades.
My guess is sunlight has a lot more to do with color. HA extends from 19N - 22N; FL in the Keys is 24N and St. Pete is at 28N. You get more consistent sun, have a more consistent climate, and do not get those *&^%$#@! winter cold fronts the way we do on the mainland.
That's one drop-dead sexy looking croton whatever the reasons.