Minecraft wiki entry for particle, associated text: Exception 1: When <name> is entity_effect or ambient_entity_effect, a <count> of 0 causes the <delta> and <speed> to act like RGBE values instead, ranging from 0.0 to 1.0. The three <delta> values give red, green, and blue components for the color; the <speed> provides an exponent that makes the colors brighter or dimmer, with 128 being a default.
Exception 2: When <name> is note, a <count> of 0 causes the first value of <delta> to specify the note's color, ranging from 0.0 to 1.0. The values of 0.0 and 1.0 produce green particles, and the values in between produce colors according to the table on Note block § Notes.
Exception 3: When <count> is set to 0, the <delta> instead acts as motion values for the particle, with <speed> acting as a multiplier. <dx>, <dy> and <dz> instead becomes <motion:x>, <motion:y> and <motion:z> respectively. Particles that don't have any motion to begin with are not affected by this (Example: barrier).