This is not always the best way to identify Costus species, but it is one characteristic that many people think of when describing a plant, so I am including it in my descriptions and in the Costus Identifier Tool.
If you are using the Costus Identifier Tool, this is how I am applying the flower color characteristic: Flower color is the predominent color of the corolla lobes and labellum, taken together, and naming the color that is closest to the overall appearance of the flower. If the botanical description includes more than one flower color, I have entered multiple colors in the GingersRus database, and the results will include species with any of the listed colors. Selecting "white" will produce results including white flowers, white and yelllow flower, white and red flowers, etc. A query on flower color will also return any species where the flower color is unknown to me and has been entered in the GingersRus database with a "U". Costus flowers with spreading labellums frequently have distinct reddish veins on the lateral lobes, so I have added two other choices - white with reddish veins and yellow with reddish veins. If you select one of those 2 options, only species that include that narrower description will be returned.

Here are some example images showing how I have defined flower colors:
Flower Colors