For a while ive noticed some html color codes don' t work. For instance this is meant to be slategrey (a kind of mid grey). But when it put it on my spell it comes up as light blue.

    tick:
    - particle: redstone
      color: "708090"
      particle_count: 3
      particle_offset_x: 0.175
      particle_offset_y: -0.175
      particle_offset_z: 0.175
      location: target

I can't remember if this makes a difference, but have you tried #708090 with the hashtag in front?

Please re-open if you're still having issues here.