How to color text smoothly or how to color part of a letter?

Hello everyone! Im currently making a text that is colored when filler`s end reaches it.
Here is a link to a gif that i made:

As you can see i use html tags to color individual letters, but i can only color the hole letter, is there any way to color a part of letter?
Should i read more about shaders or where should i look for? Thanks in advance.

P.S. How to insert a gif to unity answers? When i try to do that, it says that This attachment is not permitted because the file type is invalid.

the simplest solution would be to have the normal and the colored text on top of each other and just mask the colored text as you would do with a loading bar. don’t know the name of the unity component right now, probably slider…