As a way of giving of giving more reasons for war and making colour spheres more relevant how about the following:
Colour Sphere Claims: Having nations only being able to get their colour bonus if their alliance has put in a successful "claim" to their sphere, with claims being a limited asset. The number of claims could be number of nations in game/200 rounded down. Whilst 1 claim enables the biggest 50 nations of an alliance to claim their colour bonus, 2 claims enable 100, 3 claims enable 150 etc. etc. So even if everyone coordinated perfectly (never going to happen) at best only 1/4 of nations in the game could get a colour bonus at any one time.
Claims could get divvied up amongst the colour spheres evenly with any fractions handed out to the biggest spheres in order of membership size.
Claims would be won by size of alliance membership, with two complications: 1) any alliance can put in more than 1 claim, but doing so acts as a division of their membership number. So as way of example a 90 member alliance could put in 3 claims, which would in effect reduce their membership score to 30, but if they still won it would be worth 3 claims which would worth a colour bonus for 150 alliance members. 2) any alliance can put in 0 claims at all, effectively therefore being a Grey alliance. I could also see this being used as a particularly cutthroat peace term, likely to cause plenty of bad blood and therefore good to stoke more vendettas.
One addition (although have mixed feelings about this) would be to create a new in-game treaty called colour sharing in which alliances have the option to pool membership numbers for claims or extend in-game protectorates to do the same. This might be in danger of inciting peace rather than war however
i.e. at time of writing they are 8251 nations, so 41 claims. That's 2 claims per colour + an extra one for each colour sphere other than Lime (due to currently being the smallest) When the game hits 8400 nations Lime would get it's 3 claim.
As a hypothetical say Pink had 4 major alliances which consisted of A: 80 members, B: 60 members, C: 30 members and D: 20 members. If each alliance put in 1 claim each (which I suggest is the default in-game if not selected) then alliances A, B & C would each get a claim worth colour bonus for 50 members each. Alliance A notices however that it could do better and puts in two claims. That divides it's "claim strength" to 40, still enough to win so Alliance A now gets 2 claims (enough for all it's members) Alliance B still gets one and Alliance C has it's nose put out of joint by now having no claims at all.
Lucky for Alliance C they've got big friends elsewhere on Orbis and so start a war against Alliance A for the express purpose of getting their claim back. Alliance C + friends win, but then get greedy and instead of forcing Alliance A to drop 1 claim via peace terms they tell them to drop all their claims.
So now Alliance B, C & D get a claim each whilst Alliance A aren't claiming, which of course leaves Alliance B with a decision. Does it now stick in 2 claims and risk Alliance D's ire or just stick with the one claim to avoid Alliance A's fate?
All hypothetical of course but hopefully it shows how the above might help promote conflict, war and politics over peace