Hard is good as it means you are still in mostly carboxylic acid form. Drying material partially decarboxylates it, and decarboxylated oil is darker than carboxylic acid.
Oil always turns darker with thicker films, because of the Beers Lambert effect. One layer may look like transparent stained glass, and two layers will be many shades darker and almost opaque. Bragging rights shatter pictures are always taken in thin films.
The lightest shatters are from young Sativas, extracted still green but fresh frozen.
A vacuum chamber will allow you to purge at lower temperatures, to maintain lighter colors.