We can't really know that can we. The only comparable options you have in real life is to compare Souness treatment of comparable players or ethnicities during commentary. You can make a real life comparison to yourself: Are you more positively inclined to athletes from your own country despite skincolor? Of course you are, we all cheer for our athletes, white, black, purple or pasta.
If his comments on players are not similar, reasonable people will judge that the reason Souness doesn't give Pogba praise, is because Souness doesn't like Paul Pogba. The only reason I'm ever entertaining this discussion is because the quetion of racism is an ever important one, but you're dilluting the issue by labeling any perceived issue between a white and a colored man as "possible racism". Graeme Souness is a man that plays favorites, because that's who he is. He's the farthest thing from a Manchester United fan that you can come, and along with it comes a lot of cognitive bias. He's Liverpool through and through.
For example, this is what he had to say after a match between Chelsea and Manchester United at Old Trafford in 2018: “Kante ran the midfield last week at Old Trafford,” Souness said on Sky Sports.
Chelsea lost 2-1. Pogba had a very good game. Even had a heel-tunnel on Kante.
Here's Souness after last weeks opener against Leeds:
Micah Richards: "Pogba had four assists"
Graeme Souness: "We expect that from a £100m man"
Or on Greenwood: "They have signed good players. For me, the star of the show was Greenwood. For a young boy, I thought he was sensational." - Greenwood, as you may have noticed, has an amazing natural tan.
There's even incidents where Souness has praised Pogba, but not without dropping some hints over why he might not actually like Paul Pogba.
''I thought he was really good. Apart from this [the penalty run-up]. This for me is 'look at me, look at me, I want to be in the spotlight'.
''I just don't get this penalty, but I think he's moved the ball quickly today, he's been uncomplicated... I think he's been really good today.''
Even more commentary in 2018 saw this praise/criticism in one:
"This boy has been blessed with the most wonderful athleticism, he's six foot four, I'm sure he can leap like a stag and run like the wind. He's got fabulous technique, he's got fabulous athleticism, but for me he just doesn't do the basics right.
"But there is a player there. I'm seen as his biggest critic but his manager is not happy with him. I've been a manager and I'm saying to myself, like Jose will be saying to himself, 'what a player this boy could be if you would just listen and learn.'
''He should be turning up every day listening to the manager, listening to the senior players and wanting to improve. I’m not sure if that is the case but I’ll come back to it, this boy could be a top, top player.''
In summary, and I'll keep it breif: Graeme Souness doesn't like Paul Pogbas personality and off field antics, and is unable to stop it from seeping into his commentary. He's not a racist, he's just a dick.
Please stop looking for reasons to bring race into discussions when there is none. Let's stop watering down a very important issue with imaginary reasons, and especially let's stop bringing speculation into what "may be the reason", it does no one any favors.