Github Readme.md Color Box

Question

I want to add a color box to the README.md in my GitHub repository

In the beginning, the below worked:

![#243B55](https://via.placeholder.com/10/243B55?text=+) #243B55

But after the new API, the below works for some colors but not for all:

![#f03c15](https://via.placeholder.com/15/f03c15/f03c15.png) `#f03c15`

How can get a code for all colors?