Himaruya posted this colour chart on his blog some time ago.
Links for the official image and the translated version:
[official blog post] [Hetalia LJ thread 01] [Hetalia LJ thread 02]
He did it to be used for reference, but that does not mean you necessarily have to use these specific colors. Take in consideration that even Himaruya colored the same characters with different shades and colors over time. Sometimes, he may simply have opted for a softer color palette for a particular drawing; he may be studying new colors in order make the character looks more like the way he imagines him/her; or he may have simply chosen certain color because it was more in harmony with that particular drawing than another color.
So, if the artist likes pastel colors, or bright color, or whatever, it’s a decision that rests solely to the artist. I believe that constructive comments or questions are important and can help the artists to evolve. You can indicate references and give your opinion and simply try to better understand the artist.
But the same will not happen if you just say “argh! Your art is bad and racist! His eyes are green and not blue!!!”. Remember that everyone evolves with practice! Encourage the artists to continue practicing, and don’t make they lose the will to draw or leave the fandom just because you don’t like the way they draw today.