This is the tween variable declared in beginning, and Tween as in Node is added under Progressbar Node :
onready var tween = get_node("Tween")
This is the code for changing color that i found on questions.
if time > 0 && time <= 25:
#Attempt to change color of progress bar as time goes below 25 seconds
tween.interpolate_method(get_node("Control/ProgressBar").get("custom_styles/fg"), "set_bg_color", Color("0081ff"),
#Color("ffffff"), 1, Tween.TRANS_LINEAR, Tween.EASE_IN_OUT)
This is the code that i put in the time constraint, in so the color changes as time in progress bar goes under 25. Thanks.