Very strange feature

Hi to All, thi is my first post

I want to make a very strange and hard feature, i tried a lot of solutions but any works.
I'm going to explain my problem :

As I write I want to color the text by the number of characters already included.

For example :

I want to color the first three characters yellow and the remaining red.

I'm writing 12345678 than 123 is yellow and 45678 is red .

But I want to do this as I write, this means that if i insert 0 between 1 and 2 the character 3 become red because only the first 3 character are yellow.

How do I develop this functionality?

I apologize for bad English

I hope you can help .

Thank you.