Tigered stripes in Black?

[GrimsbyPatchwork]

Hi everyone,

My Jacksoni Jacksonii (willigensis) is about 10months old and is usually a dark green colour with a yellow stripe and have noticed that sometimes he sports black tigered stripes over the dark green ...any idea what it means? environment and temperatures are as suggested

 

[cuso]

Post a pic. My Jackson xanth turn dark if stressed or cold