I've been thinking a lot about how Spooky Tales, Roommates and how Yellow could have been a kid lately. I've read lots of comments and looked back at old theories.

Rocking Kitty/2muchofafangirl and CiCiTheMifwa mentioned stuff similar to some of the things I think.

Many people think that the Vacktor's really were Red's siblings at one point, and I agree. I mean Yellow was a child once he didn't sound like he was lying, but Roommates makes them out to be his emotions. However, Red only said that he thought this. I think that somehow his siblings died and they were fused together. On YouTube Rocking Kitty said she thought that Red might have been trying to bring back Magenta, and that's how they ended up fused together. If that was what happened it would make sense why there was this colorful Vacktor thing in Roommates episode 4.

I think that Red somehow accidentally ended up killing his siblings and they fused together. Red thought they were gone forever and started having emotional problems, that and the whole Doogle thing which caused him to try and get rid of his "emotions" thus bringing his siblings back to life. At first Red didn't believe they were his family and he showed no interest in them. Eventually Red learned the truth that they are his siblings and they got their memories back, which is why Yellow mentions his childhood, Blue talked about his mom, and it explains why Green, Pink, and Red feel sad about Magenta now. 

The reason they seem to act one way is well I have three theories about that

1. They're onesies were made to reflect their personality when they were younger. Which is why they seem to act one way perhaps because they feel boxed in my their symbols. Red originally had the lighting bolt which really doesn't have a personality that goes with so he has a more broad range of emotions.

2. Red told them what emotions they were and they believe they can only feel/think that way now. Yellow even said that he had no choice and that had to smile. (Either that or it has to do with the robot theory)

3. That's just their personalities. Some people are generally more optimistic, some are more quiet, some are less emotional. The Vacktor's act this because that's who they are.

I don't really think they can't feel other emotions, because we do see them express other emotions at times (I wrote about that earlier)

CiCiTheMifwa made a comment about how she thought the Vacktor's were acting this way because of Magenta's death. I think this may also be a possibility as to why the Vacktor's act this way. 

As for how Spooky Tales existed and as to why Barney doesn't remember Green, Rocking Kitty also commented about how it could be due to the experiments Red does on Barney.

I also think it's strange that Red had a room titled warning room contains high density purpleness, or how Yellow knew he loved the sun and video games or why Blue was always left for last if they were just created. 

I hope I explained this well enough. 

I've been thinking about writing some fan fiction about these theories and about the Vacktor family, but life is very busy right now, so hopefully I'll be able to write something in the future :)