You also can produce Calcium Phosphate nanoparticles with light blue color using these sources. I think the reason might be due to the time and temperature which you use to synthesize your nanoparticles.
there is the possibility of going to the blue color in the case of synthesis of nanoparticles in the presence of polymers: nano composite, but when heated to 900 ° C for crystallization, it turns white
I synthesized nano-HA at 600'C for 5h with a heating rate of 10'C/min and my sources were Ca(NO3)2.4H2O and P2O5. The color of my synthesized nano-HA was bluish.
I justify it because I repeated this procedure several times.
I have made second preparation of calcium phosphate nanoparticles with same method showing white colour. Moreover in literature white pellet of calcium phosphate nanoparticles is reported.
Can you send me your research paper/s about which you are telling me?