Mine are sometimes like that too.
Usually males. Losing and gaining back their color.
I think males by nature aren’t as concentrated in color as females.
This also led me to theorize some culls are actually unnecessary.
That culled shrimp may regain it’s color. It’s just in a temporary state of being a juvenile, or malnutrition, or lacking pigment.
Kumoshojo 
Mine are sometimes like that too.
Usually males. Losing and gaining back their color.
I think males by nature aren’t as concentrated in color as females.
This also led me to theorize some culls are actually unnecessary.
That culled shrimp may regain it’s color. It’s just in a temporary state of being a juvenile, or malnutrition, or lacking pigment.
Alotta fish and invertebrates lose colour at night, its normal.
some lose color when they are going to sleep to camouflage