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Callas can indeed be grown from seed. Once seed development begins, the flower fades. You will notice the “bumps” on the calyx enlarging; the weight will begin to pull the stem to the ground. This is normal. In time the seed capsules will begin to turn yellow and become mushy to the touch. It appears it's beginning to "rot" but this is all part of the process. At this point, the seed are mature, and each capsule contains at least one or two seeds, each resembling a miniature tuber, about 2 mm in size. Harvesting the seed is a bit messy; just gently squish out the individual seeds. Plant the seed 1/4" deep, space them about 1/2" apart. Cover lightly, and keep them warm, well lit & moist. Germination is erratic, and could take several weeks. Transplant after they've grown two or three leaves. Under optimum growing conditions, you might get a bloom in the second year; otherwise, expect something in the third year.

Arum lilies can sometimes be tricky to get to rebloom.  This article will help you (Calla is another name for Arum): https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/make-calla-lily-bloom.htm

Here is an explanation for why the colored flowers change to green or a streaked green color. Calla lilies are a type of arum lily.
https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/green-calla-lily-blooms.htm

There are also varieties that change color as they mature; This is a varietal trait.

This plant seems to be rare and difficult to track down. Your best bet might be to contact gardening societies and clubs in your area and ask whether anyone has this plant. Or, search on the internet for a bulb from a reputable dealer. They are supposed to be difficult to grow from seed.

This might be an Arum maculatum:
http://symbiota4.acis.ufl.edu/seinet/vplants/portal/taxa/index.php?taxon=73226

This is a Dragon Arum.

https://www.gardeningknowhow.com/ornamental/bulbs/dragon-arum-lily/growing-dragon-arums.htm