You probably need a good WhiBal like reference card.
You have to check the color temperature of the arena lights. If it does not match the flash you might have to gel it.
Does the 420EX do high speed sync? You might have to darken the ambient exposure and use the flash in HSS mode to overpower it.
If the arena lights are solid state LED, there is a high probability that they will be flickering at line voltage frequencies.
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