Actually, girl isn't saying "she is red". It would be "îíà êðàñíàÿ".
It is correct to translate "êðàñíàÿ" as red, since 'red' doesn't have gender in English.
Unlike that, in Russian there are 3 singular forms:
êðàñíûé - "(he is) red"
êðàñíàÿ - "(she is) red"
êðàñíîå - "(it is) red"
All three would translate 'red'.