No idea. Bugs me too, haven't been able to figure out how to change it.
That's normal because you define mc as:
Python:
define mc = Character("Thomas", color="#FFD700")
But at the start of the game you do:
Python:
$ mc = renpy.input("What is your name? Default: Thomas")
$ mc = mc.strip()
if mc == "":
$ mc="Thomas"
You use the same variable for 2 different uses. In the end "mc" isn't a Renpy Character but just a string. That's why the default yellow color is used.
The right way to do would be:
Python:
define mc = Character("[mcname]", color="#FFD700")
default mcname = "Thomas"
label start:
$ mcname = renpy.input("What is your name? Default: Thomas")
$ mcname = mcname.strip()
if mcname == "":
$ mcname = "Thomas"
(You should also store all input variables in persistent variables if you plan to add a gallery later)
You can also add the attached file in your game if you want to fix that in old saves.
Edit: Too late it seems
![Whistling :whistle: :whistle:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)