unlike the other parts of body,these parts like inner thigh/underarms are darker in colour.so if the darkness you saw is after fungal infection,it can gradually be reduced by treatment whilethe actual colour remains more or less same.
Next Steps
you can apply 10% aziderm cream at night on the pigmentation area.it will also reduce post infectious inflammation.
Health Tips
friction increases pigmentation.avoid friction(weight reduction helps in reducing friction)
alao avoid precipitaion of sweat and clean the areas properly with soap/body wash daily and keep it dry.all these measures also help in reducing pigmentation