I used rainbow neos before. It was fun but not much draw power if i should say..
If you want to use a rainbow neos deck, then you should concentrate on rainbow neos more than other fusions..
Calculator +1/+2
Neos +1
Prisma +1
Rainbow dragon +1
Oversoul +1
MST +1
Torrential +1
Dark rainbow -1
Honest -1
Woodman -1
Sparkman -2
foolish burial -1
And the main reasons here are the following: As judge stated up , its useless using prisma sending rainbow cause of rulings. Now, the main strategy here is, getting stratos, adding prisma to hand, then summoning prisma, sending neos to the grave , and if you got rainbow , activate fusion, however, if you would like aggressive play, after sending neos to graveyard, activate oversoul adding him to the field, so basically you got 1700 (prisma) 2500 (neos) and stratos if possible. Just wait for rainbow to come up, then you use poly... Once you get rainbow neos on the field, simply summon
Calculator (lvl 2) :
This card's ATK is equal to the combined Levels of all monsters you control x 300.
Rainbow neos= 10 stars + Calculator = 12 stars ,, 12 stars x 300 = 3600!!
So you got a 4000 attack monster and a 3600 attack position!! What more you want? Simply get any level 1 on the field, destroy it by rainbow neos, return all monsters then go for the otk..
HOWEVER, this deck is mostly, luck draws and you will win with it and be happy, but you will lose more than you do win and you will leave it after some time... Its not stable and a simple bottomless can destroy your dreams.