Minecraft wiki entry for Firework_Explosions, associated text:  Colors: Array of integer values corresponding to the primary colors of this firework's explosion. If custom color codes are used, the game renders it as "Custom" in the tooltip, but the proper color is used in the explosion. Custom colors are integers in the same format as the color tag from Display Properties.
 FadeColors: Array of integer values corresponding to the fading colors of this firework's explosion. Same handling of custom colors as Colors. May be absent.
 Flicker: 1 or 0 (true/false) - true if this explosion has the Twinkle effect (glowstone dust). May be absent.
 Trail: 1 or 0 (true/false) - true if this explosion has the Trail effect (diamond). May be absent.
 Type: The shape of this firework's explosion. 0 = Small Ball, 1 = Large Ball, 2 = Star-shaped, 3 = Creeper-shaped, 4 = Burst. Other values are named "Unknown Shape" and render as Small Ball.