Update: Use libc functions more in f_color.

Update: Use libc functions more in f_color.

The f_string_dynamic_increase_by() guarantees the size is allocated.
The f_string_dynamic_append() doesn't need to be called when the simple memcpy() can be directly called.

Doing this saves the status check and the additional function calls.
This likely increases performance but this performance increase potential has neither been tested nor confirmed.