• 白あん needs to be sufficiently soft
• I used 20 gr 中あん, 15 gr outer white あん and then 8 gr of colouring. I made 8 ╳ 1 gr balls of different colours, but I guess 4 ╳ 2 gr is just as well (maybe even better because there's more clarity).
• Wrap colours in white あん, then press down and wrap 中あん. For pressing down, using a wooden board might be a good idea, to get a very symmetrical disc.
• Make indentations like with 菊, they can also be quite pronounced.
• Then from the middle of a section push outside and repeat for every other section. For the unpushed sections then do another round, pushing slighly less.
• I got the following tips for next time:

For this decoration, you need to place the groove of the shaping tool straight through the centre. You should also pay attention to the angle at which you insert the spatula.
And to make it look even more like fireworks, vary the lengths of the lines—some long and some short. That will make it look as though the fireworks are bursting open.
For the lower part of the two-tiered design, if you press a little more firmly with the spatula and make those lines longer, I think it will change the overall appearance and impression as well.