Specifies the size of the 3-dimensional cuboid volume to spawn particles in, centered on position x y z, and divided by about 8 (using 1 1 1 specifies a cuboid of about 8×8×8 in size).
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Exception: When the particle chosen is <b>reddust/dust</b>, <b>mobSpell/entity_effect</b> or <b>mobSpellAmbient/ambient_entity_effect</b> and speed is not 0, and count is 0 or not specified, these values are set as the RGB channel values of the color of the particle, and are specified in the range 0 to 1. In this usage, the speed parameter acts as a multiplier on these values. If count is greater than 0 for those particles, xd yd zd are treated as a 3-dimensional volume as usual, rather than as a color, and randomized colors are used.
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With <b>mobSpell/entity_effect</b> or <b>mobSpellAmbient/ambient_entity_effect</b>, 0 0 0 produces a black particle for example, 0 1 0 produces a green particle, and 0 0 0.5 produces a dark blue particle, etc.
The <b>reddust</b> particle's RGB channels behaves differently. The Red Value(xd) should only be set to 1 or 0.001. 
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The color chooser below will not accurately show the color you get when using <b>redust/dust</b>. But should work for <b>mobSpell/entity_effect</b> or <b>mobSpellAmbient/ambient_entity_effect</b> provided speed is set and count is 0 or unset.