数式

sum(split(prop("入力（16進数）"), "").map(
	ifs(
		current == "A", 10,
		current == "B", 11,
		current == "C", 12,
		current == "D", 13,
		current == "E", 14,
		current == "F", 15,
		current.toNumber()
	) * pow(16, length(prop("入力（16進数）")) - index - 1)
))

Step By Step

① 文字列を１文字ずつの配列にする

split(prop("入力"),"")

② １文字ずつ文字を数値に変換する

split(prop("入力（16進数）"), "").map(
	ifs(
		current == "A", 10,
		current == "B", 11,
		current == "C", 12,
		current == "D", 13,
		current == "E", 14,
		current == "F", 15,
		current.toNumber()
	)
)

③ 位に合わせて16の累乗を掛ける

split(prop("入力"), "").map(
	ifs(
		current == "A", 10,
		current == "B", 11,
		current == "C", 12,
		current == "D", 13,
		current == "E", 14,
		current == "F", 15,
		current.toNumber()
	) * pow(16, length(prop("入力")) - index - 1)
)

④ 合算する

sum(split(prop("入力"), "").map(
	ifs(
		current == "A", 10,
		current == "B", 11,
		current == "C", 12,
		current == "D", 13,
		current == "E", 14,
		current == "F", 15,
		current.toNumber()
	) * pow(16, length(prop("入力")) - index - 1)
))

使用例

16進数のカラーコードを10進数にすることができます。
先頭の#は、toNumberメソッドで値を返さないので無視されます。

参考