Aesculs x carnea has just plain pink/light red flowers. This is the sterile one that doesn't produce fertile or many chestnuts.

The normal Horse Chestnut produces white blooms, but look closely and they contain spots of pink and yellow.

Atleast i assume you are refering to different plants..!

if you look closely at Aesculus hippocastanum its flowers are beautiful with pink and yellow in each floret and I'm sure you know it got the name horse chestnut from the horseshoe markings on the branches

I was told too that its called HIPPOcastanum as apparently HIPPO is Greek for Horse. Never checked thou to see if thats true...

hippo is greek or is it latin for horse. one is Hippo as in hippopotamus [river horse] and the other is equs as in equine.

The alternating pink & yellow markings on the white flowers is really pretty too.