They might self-sow, they might survive the winter outside on/in the soil, and they might or might not cross and produce more colors. If the two plants you have are two different colors, you might get the same two colors or a cross of the two. If your neighbors are growing portulaca and the pollinators cross, you may get more colors.
So many "if's". Short answer is, I really don't know. I've always bought transplants of portulaca in the colors I wanted, and they have not lived long enough for me to self-sow. They really do not like our heavy summer humidity.
Cover your bases and collect some seed if you can. Also try starting more plants from cuttings if your plants get big enough. With their lax, spreading habit I'm pretty sure they will root at the leaf nodes.
How are the chrysanthemum cuttings doing?
