Haha how is there not??? If you use a white element, it adds white to the field status, weakening all black elements. If you use Serge to cast 1 or 2 white elements, then whatever black element you cast will be weaker than it would have with no white in the field effect. Having 2 members with opposite innate colour in your active party is a blunder.

It only gets stronger if the whole field is one color. I'm pretty sure Radius teaches you that in game.

Join a Chrono forum and learn something

Play the game and learn something. Even if it did decrease the damage, its not even noticeable. i can use diminish and full cure in the same group. Two elemets on the field doesnt do shit unless its all one color. Try using the same black element on Lynx after he uses some white. The game is easy enough as it is anyway, id rather use my fav character than some shitty yellow because you think it would add an extra 20 dmg.

Any black in the field effect will lower white elements. Diminish isn't that useful even on the first time around, let alone in new game+.
White, Blue, Green is an awesome combo. Esp with Glenn + Serge you then have the option of using X-Strike tech which is the strongest red element in the game vs blue innates.
You probably don't know this, but the game's 6 elements are split up into 2 groups of 3 - light and dark (these effect the evolution of pip especially). And even just using any of the dark elements will effect the damage of your light elements and vice versa.

No matter who your favourite character is, it can't be argued that the perfect party would have a black and white innate both active at once.

By full cure I'm going to venture a gues that you either mean Full Revival or HolyHealing... both of which are amazing. Diminish however is not.