can we change the colour of "Forgot your password?" in the cognito UI login page ?

To change color, try something like this, in the CSS stylesheet that you can upload to theme the login page:

a.redirect-customizable {
    color: red;
}

To replace the link, you can also do that in CSS:

/* set font size to 0 */
a.redirect-customizable {
  font-size: 0;
}

/* add text with the original font size */
a.redirect-customizable:after {
    content: 'Text of the link';
    font-size: 14px;
}

You can also combine both:

a.redirect-customizable:after {
    content: 'Text of the link';
    font-size: 14px;
    color: red;
}