Answer: Based on your description I am not certain what has happened to your Syngonium but I suspect the overwatering may have caused root damage and the discolored foliage is a continued symptom of that. If there is a healthy stem you may want to try to root it as a precaution in case you lose the whole plant.
In the meantime, try to optimize the growing conditions for it: normal room temperature with high humidity but also good air circulation, keep it away from hot or cold drafts; bright light but indirect light (this plant tolerates low light but does not prefer it); water when the soil begins to dry out a bit, make sure to empty the saucer beneath the plant after watering, if there is one.) Since you have just repotted it, still take care not to overwater. Do not overfertilize -- use a water soluble fertilizer at the lower rate on the label every few weeks from spring through fall, do not fertilize in the winter months. If your potting mix included a fertilizer you may not need to feed for several months after repotting.
Check it for fine webbing along the stems or undersides of the leaves, if you find these then there is a problem with spider mites. These can cause a yellow/brown stippled discoloration on the leaves. Treat them with commercially formulated insecticidal soap per the label directions. Or, rinse the plant in tepid water twice a week (as long as there is no fungal or bacterial problem, see below.) Increasing the humidity level helps control spider mites as they thrive in the dry indoor air during the winter.
If you think the spots are fungal or bacterial, remove the affected foliage and avoid spreading it by not touching the healthy leaves; do not mist the plant; increase the air circulation and do not crowd it with other plants.
Some of these plants have variegated foliage including some red or bronze beginning at the stem and spreading out the leaf. If it is receiving more light than it did before, some of the red color you are seeing may be related to that because more light will intensify the coloring. (In dim light, the variegated types show more green and much less of the variegated coloring.) Also, if the plant had not been fertilized for a while it might not have developed its full color.
I hope this helps -- good luck with your plant!
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