can this code be considered as idiomatic C#?; considering todays standards.

Time complexity of integer square root is log(n).
And then you are iterating that many times.
So, the time complexity is θ(log(n)+n)

Whereas using take while, the time complexity is θ(n)

What about this input string: "4.7 ohms"?

From the description:

For resistors less than 1000 ohms, the ohms value is just formatted as the plain number.

The only possible variants with 4.7 then are:

encode_resistor_colors("4.7k ohms") // yellow violet red gold = 47 * 10^2 ohms
encode_resistor_colors("4.7M ohms") // yellow violet green gold = 47 * 10^5 ohms

I hope that helped.

Have a great evening.