Browning or a dull color is caused by oxygen in the jar. Make sure you don't leave too much headspace, and that you try to get any air bubbles out of the jars prior to processing. You can run a clean skewer, butter knife, or chopstick around the edges of the jar to remove air bubbles. Some recipes will have you add a teaspoon of butter to prevent foaming (foaming = more air bubbles). You could also add citric acid to prevent browning.