Sometimes I want Six to remember Rex, but since there is no way to return the memory, I thought that the jump in the past can arouse feelings of fatherly love, because I think that the age and vulnerability of Rex could cause these feelings
I think you do not need to say who can travel back in time
EDIT
I saw a new episode "A Brief History of Time" and I felt sorry for that time-machine was destroyed but nevertheless I was able to colorize this picture
This is a beautiful picture. I love how you did the coloring and lighting; creates a "bitter-sweet memory" feel. Everyone's expressions are amazing; Rex looks peaceful, yet vulnerable, past-Six looks protective yet solemn, and future-Six looks like he's feeling a variety of emotions all at once.
Yeah..... Totally fangirling over the two Sixs right now. Awwwwww.... This is so sweet! You have to admit Six makes for an awesome mother/father figure.