There is no button to change it. That color gets generated automatically. In fact it remains at the same state, even after the loco modification. I can show you a similar example. 12369 / 12370 Kumbha Express. Earlier it used to undergo a loco change, but currently it gets end to end electric loco. Still the loco color hasn't become fully green.
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I am quite sure, no Admin or TL Monitor will be able to help you here. Please contact Moderator about this issue.