Monocolour regional isn't about the players but is also about the team skills.
Basically, some players have team skills with 15% to all stats.
It's usually like 15% to blue Japanese or
15% to red south Americans.
If you get THREE players who have these team skills then you can (for example) get 45% to Red South Americans.
If you get 45% to all stats for players of a certain colour and nationality AND you have enough correct colour and nationality players to fill this team (esp GK and FW), your stats will be boosted the most by team skills.
That's the strongest set up now:
45% to all stats for players of a certain colour and certain nationality.
Monoblue Japan is considered strong because it has enough strong players to fill 11 spots. It also has DF Ken and DF Hyuga (top GK and FW).
Some colours don't have enough players.
For instance, there is NO Red South American DMF. So it's not as strong as Monoblue Japan.
Other colours don't have enough STRONG players.
For instance, there is no STRONG Red European GK. (not DF level). So Monored Europe isn't considered complete either.
Note that if you play all blue Japanese players but WITHOUT the 45% to all stats, it's just not strong at all. It won't be considered a "true" Monoblue Japan.
So it's not just the players but it's the team skills AND players.
Hope it helps!