Using multiple white LEDs to obtain a unique CCT
White LED's are typically created with 2 phosphors that absorb some of the intense narrow blue spectrum and convert to many Gaussian distributed longer wavelengths. The phosphor thickness is in microns is difficult to control compared to the huge thickness of the Blue LED. So originally they could only make cool or warm. Now they can control the process to get ranges in between like ANSI white or Neutral White or 4500'K or 5000'K

But this doesn't tell the whole story. It only shows what the apparent colour is to our eyes and not the observed reflectance on pastel colours which defines the CRI index. This because White LEDs are missing a lot of the spectrum. 

There is another NIST standard, not adopted widely, called CQS which shows the colour quality for intense saturated reflectance. This makes LED's better than FL tubes so the strong FL lobby industry did not want this standard to be adopted.

In short if you only wanted to cool Warm LEDs, you only need to add sufficient Blue LED Light so the peak blue and Warm phosphors are  about the same intensity to get 4000~4500K.  

But you can't easily make a cool LED to a warm LED  since there are no LEDs with broad spectrum but you can approximate it with a calibrated instrument  using RGY LEDs.

Samsung


Cree