Is the wrapper split ? If it's split, STOP using it immediately and re-wrap. Or better, buy some new ones.
If it's just discoloured, if the battery is old or been used on many cycles, e.g you vape like a 40 a day smoker, it's more than likely the chemical elements inside have began to degrade.
If I was you, recycle these and buy some more. Authentic VTC's aren't ridiculously expensive and with battery's, you always want to be more safe than sorry.