The colours are not soft as such, though they are pastels; but the shadows are simply a darker version of the "original", and there are careful use of highlights. Consider the clothes:
The outlines are trad cartoon-black. If you removed the shadows and kept the black outlines, you would have old-school-western donald-duck-style cartoon.
Really old Donald Duck, has the shadows as an "overlay" of gray-to-black:
"Modern" (well, to me anyway) versions do the same as manga: shadows are just darker colours of the highlights, hence the softness:
In response to some of the comment; I actually do not see a blur. What I do
see, is outlines that are not entirely
black, and in some cases, the outline colour is adjusted to fit the "content" of that object.
Note that some of the outlines are brown-ish, others are blue-ish etc.
I could of course be wrong.