The tag works fine for mobSpell, but not for any other colourable particles, e.g. mobSpellAmbient or reddust. In the last 2 cases, it just shows randomly coloured particles.
EDIT: On 1.11.2, it doesn't show randomly coloured particles, but instead shows particles of a default colour no matter what the Color tag is (e.g. red reddust particles, greyish mobSpellAmbient particles)
EDIT 2: In 17w13b, the randomly-coloured particles are back instead of the default colours