In a heatproof bowl, whisk the shiratamako, sugar, and water together until smooth with no lumps.

Set the bowl over a 1.5-quart / 1.4 L saucepan of gently simmering water. Stir constantly with a silicone spatula for 5 to 6 minutes, until the cloudy mixture turns glossy, translucent, and pulls together into a soft, sticky mass.

Remove from the heat and let it cool for 2 to 3 minutes, until warm but safe to handle.

Add the shiro-an to the warm gyuhi in the same bowl. Knead with the spatula, then with cornstarch-dusted hands, for 3 to 4 minutes until the two doughs merge into one smooth, pliable paste that doesn't stick to your fingers.

Divide the dough into portions for coloring. Keep one portion white, and knead a few drops of natural food coloring into each of the others, one color at a time, until the shade looks even.

Divide the koshi-an into 20 g balls, about 8 total, and roll each one smooth between your palms.

Flatten a 44 g piece of colored dough into a disc in your palm. Place an anko ball in the center and stretch the dough up and around it, pinching and rotating to seal completely with no cracks.

Roll the wrapped ball briefly between your palms to smooth the seam, keeping the seam side facing down.

Shape each ball into a seasonal motif. Cup it in a folded tea towel and twist gently for spiral petal lines like a chrysanthemum, or press a wooden skewer into the sides at intervals to define individual petals for a plum blossom.

Use the tip of the skewer to add a center dimple or vein detail to each piece.

Set the finished pieces seam-side down on a parchment-lined tray, dusting your palms and work surface with cornstarch as needed to prevent sticking, and chill for 10 minutes to firm up before serving.