If you have a static color change in mind, there’s a very rudimentary option.
Set a condition on the Input element: When This Input is not empty. Select Font Color as the property, set color to red (or whatever). This results in:
When Input is empty, and user is not actively typing in the element, the text color will be black (or default).
When Input has some previously entered text, even if user is not actively typing in the input, text color will be red
When Input is empty, and user begins actively typing, the text color will turn red (may be a slight delay)
Not very elegant, but partial solution. You might be able to get a workaround for #2 above by setting an additional condition “When input isn’t focused…” and set Font Color to black. This may conflict with the first condition, though, so may not be entirely reliable.