There are 3 types of ORANGE KOHAKU.

1. Just poor quality. This type doesn't improve even when given colour food.
A lot of KOHAKU have taken SENSUKE blood in an aim to grow larger koi.
The Original SENSUKE colour wouldn't improve, even when fed colour enhancer food .
For example 19th Grand Champion KOHAKU.
                                                                         
                                                *photo / 19th Grand Champion KOHAKU

2.It can keep it's deep red colour by continuously giving colour food.
Since breeders have succeeded in mixing other lineage with SENSUKE, some KOI have a quality of Hi which can be improved by feeding with colour food.
For example 33rd Grand champion 92cm SAKAI HIROSHIMA produced KOHAKU.
I think the hi on this KOHAKU would be thick orange if no colour enhancers were give.

                                                          
                                                                 *photo / 33rd Grand Champion KOHAKU

3.This will be deep red naturally when the koi gets to a mature age.
As Mike said 'traditional type KOHAKU can improve the Hi when they get to an older age, without the need to colour enhance. However, This type of KOI seldom to grow extra large!!
For example 22nd & 24th, twice Grand Champion KOHAKU.

                           
                             *photo / Young ROLAN                                          and ROLAN Champion