A super high protein diet is the best for fin regrowth. Frozen (not freezedried) bloodworms are a good start, and a high quality fish food like New Life Spectrum are your best bets. In fact, a high protein diet really is best for bettas anyways since they are carnivores. If you read your fish food ingredients and the first 2 ingredients are not some sort of whole fish (or shrimp) meal then you need to find a different pellet. They will taste better and your fish will end up healthier, thus with better color!
The next thing will be increased water changes. At least every other day, 50-100% depending on your tank size. If you have a cycled, filtered tank over 5 gallons you could do 25-50% changes.
The most important thing of all is to actually test the plants with pantyhose. Silk, or plastic, both of them have caused problems for people because they didn't do the simple test. It is the only foolproof way to make sure your fish is safe.