You can change the text color in the on Focus event, but the change won't take effect until the field loses the focus. So if it starts off grey and the code changes it to black, the text that the user types will be grey, until the field loses the focus and it changes to black.
A different approach is to place what you're calling the placeholder text on the page as regular page contents and make it grey. You can then use a custom Validate script that controls the background color of the text field, something like:
// Custom Validate script
event.target.fillColor = event.value ? color.white : color.transparent;
In addition to the following On Focus script:
// On Focus script for text field
event.target.fillColor = color.white;
