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I'm finding it useful to separate the calculation phase from the colouring phase, so I don't have to recalculate things just to change colours.

Is there any standard interchange format in use to save raw calculation results from one program and load them into other programs for colouring?

So far I've used multiple PNM image files, one for each layer:

P5 PGM 1 channel 8bit for binary interior/interior
P5 PGM 1 channel 16bit for fractional part of escape count, final angle / two pi, distance estimate relative to pixel size, perturbation error estimate
P6 PPM 3 channel 8bit for integer iteration count, minimal |z| iteration count (period) - packed 24bit int into 3 8bit bytes

This has the advantage of being easy to preview and play around with in image editors like GIMP, but it's a bit of a pain being limited in resolution (afaik GIMP doesn't 16bit) and having to repack data via multiple channels for full range.

If there's a (semi)standard format I could implement that'd be great, otherwise I might go with a home-rolled solution packing float image planes into a container with metadata describing what the data represents (I've experimented with bencoding, not got very far yet).  One 1920x1080 keyframe works out around 50MB with all the layers, stuffing it through bzip2 gets it down to 30MB...

What would you want from such a format?  Would you even be interested?

I haven't looked at the updated feature list but maybe OpenEXR 2.0?

OpenEXR 2.0 looks good, thanks!

Particularly relevant features:
32bit float image data
32bit unsigned int image data
arbitrary channel counts and names
arbitrary metadata
deep data (variable amount of data at each pixel - might be useful for partial periods)
multipart files (might be useful for animation keyframes)