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I would hold off on cutting the foliage, though you may trim back the flower stems on any spent blooms.

You can plant them out when the night time temperatures in your area are reliably above 12c.

It depends on the variety but most varieties grow to be about 30-40cm tall.

Yes, just prepare the planting hole around the same depth as the pot it is currently in and water it well. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm

They have most likely reached their peak and are now done. Generally, potted calla lilies bloom 4-6 weeks, though there is no telling how long they were in bloom when you got them two weeks ago. Blooming may also be delayed or shortened if not receiving adequate lighting. Simply remove the spent blooms/flower stalks.

They can be grown indoors or outside in the garden. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm

If you want the calla to rebloom, this article should help you with that: http://www.gardeningknowhow.com/flower/make-calla-lily-bloom.htm

The colors in flowers result from pigments produced by the plant, which can be affected by temperature and light exposure. Soil pH can also affect this in some plants.