Q:

Why won't my button change color when i hover over it pygame?

I'm new to pygame and have been attempting to create a simple interface with some buttons. I can't get the button to change color when the mouse hovers over it.
I've managed to create the button, but cannot get it to interact with my mouse.
The code create an button object with one instance of a green button.
It should change the button from green to red when mouse hovers over.
import pygame

pygame.init()

display_width = 1200
display_height = 600

black = (0, 0, 0)
white = (255, 255, 255)
red = (255, 0, 0)
green = (0, 255, 0)

StartScreen = pygame.display.set_mode((display_width, display_height))
pygame.display.set_caption('Log In')
clock = pygame.time.Clock()

StartScreen.fill(white)

class Buttons():
    def __init__(self, color, x, y, width, height, text=''):
        self.color = color
        self.x = int(x)
        self.y = int(y)
        self.w = int(width)
        self.h = int(height)
        self.text = text

    def Draw(self, StartScreen, outline=None):
        if outline:
            pygame.draw.rect(StartScreen, outline, (float(self.x-2), float(self.y-2), float(self.w+4), float(self.h+4)), 0)

        pygame.draw.rect(StartScreen, self.color, (self.x, self.y, self.w, self.h), 0)

        if self.text != '':
            font = pygame.font.SysFont('comicsans', 20)
            text = font.render(self.text, 1, black)
            StartScreen.blit(text, (self.x + (self.w/2 - text.get_width()/2), self.y + (self.h/2 - text.get_height()/2)))

    def MouseOver(self, pos):
        if pos[0] > self.x and pos[0] < self.x + self.w:
            if pos[1] > self.y and pos[1] < self.y + self.h:
                return True

        return False

def redrawWindow():
    StartScreen.fill(white)
    GrnBut.Draw(StartScreen, black)

run = True

GrnBut = Buttons(green, 150, 200, 90, 100, 'Press')
while run:
    redrawWindow()
    pygame.display.update()

    for event in pygame.event.get():
        pos = pygame.mouse.get_pos()

        Exit = False
        while not Exit:
            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    print(event)
                    pygame.quit()
                    quit()

        if event.type == pygame.MOUSEBUTTONDOWN:
            if GrnBut.MouseOver(pos):
                print("Clicked")

        if event.type == pygame.MOUSEMOTION:
            if GrnBut.MouseOver(pos):
                GrnBut.color = red
            else:
                GrnBut.color = green

A:

Your main problem is that you have a nested event loop inside your event loop:
while run:         # outer loop
    redrawWindow()
    pygame.display.update()

    for event in pygame.event.get():
        pos = pygame.mouse.get_pos()

        Exit = False
        while not Exit:       # inner loop
            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    print(event)
                    pygame.quit()
                    quit()

When execution reaches this inner loop, neither redrawWindow() or GrnBut.MouseOver(pos) is ever called again.
Just get rid of it:
while run:
    redrawWindow()
    pygame.display.update()

    for event in pygame.event.get():
        pos = pygame.mouse.get_pos()

        if event.type == pygame.QUIT:
            print(event)
            pygame.quit()
            quit()

Your code can be improved by using some of pygame's features, such as the Sprite and Rect classes.
Here's an example of how you could create a more "pygamy" version of your Button class that supports multiple, different buttons:
import pygame

pygame.init()

display_width = 1200
display_height = 600

# use python style variable names (lowercase)
screen = pygame.display.set_mode((display_width, display_height))
pygame.display.set_caption('Log In')
clock = pygame.time.Clock()

# load the font only once instead of every frame
font = pygame.font.SysFont('comicsans', 20)

# class name should be singular
class Button(pygame.sprite.Sprite):
    # 1) no need to have 4 parameters for position and size, use pygame.Rect instead
    # 2) let the Button itself handle which color it is
    # 3) give a callback function to the button so it can handle the click itself 
    def __init__(self, color, color_hover, rect, callback, text='', outline=None):
        super().__init__()
        self.text = text
        # a temporary Rect to store the size of the button
        tmp_rect = pygame.Rect(0, 0, *rect.size)

        # create two Surfaces here, one the normal state, and one for the hovering state
        # we create the Surfaces here once, so we can simple blit them and dont have
        # to render the text and outline again every frame
        self.org = self._create_image(color, outline, text, tmp_rect)
        self.hov = self._create_image(color_hover, outline, text, tmp_rect)

        # in Sprites, the image attribute holds the Surface to be displayed...
        self.image = self.org
        # ...and the rect holds the Rect that defines it position
        self.rect = rect
        self.callback = callback

    def _create_image(self, color, outline, text, rect):
        # function to create the actual surface
        # see how we can make use of Rect's virtual attributes like 'size'
        img = pygame.Surface(rect.size)
        if outline:
            # here we can make good use of Rect's functions again
            # first, fill the Surface in the outline color
            # then fill a rectangular area in the actual color
            # 'inflate' is used to 'shrink' the rect
            img.fill(outline)
            img.fill(color, rect.inflate(-4, -4))
        else:
            img.fill(color)

        # render the text once here instead of every frame
        if text != '':
            text_surf = font.render(text, 1, pygame.Color('black'))
            # again, see how easy it is to center stuff using Rect's attributes like 'center'
            text_rect = text_surf.get_rect(center=rect.center)
            img.blit(text_surf, text_rect)
        return img

    def update(self, events):
        # here we handle all the logic of the Button
        pos = pygame.mouse.get_pos()
        hit = self.rect.collidepoint(pos)
        # if the mouse in inside the Rect (again, see how the Rect class
        # does all the calculation for use), use the 'hov' image instead of 'org'
        self.image = self.hov if hit else self.org
        for event in events:
            # the Button checks for events itself.
            # if this Button is clicked, it runs the callback function
            if event.type == pygame.MOUSEBUTTONDOWN and hit:
                self.callback(self)

run = True

# we store all Sprites in a Group, so we can easily
# call the 'update' and 'draw' functions of the Buttons
# in the main loop
sprites = pygame.sprite.Group()
sprites.add(Button(pygame.Color('green'), 
                   pygame.Color('red'), 
                   pygame.Rect(150, 200, 90, 100), 
                   lambda b: print(f"Button '{b.text}' was clicked"),
                   'Press',
                   pygame.Color('black')))

sprites.add(Button(pygame.Color('dodgerblue'), 
                   pygame.Color('lightgreen'), 
                   pygame.Rect(300, 200, 90, 100), 
                   lambda b: print(f"Click me again!"),
                   'Another'))

while run:
    events = pygame.event.get()
    for event in events:
        if event.type == pygame.QUIT:
            pygame.quit()
            quit()

    # update all sprites
    # it now doesn't matter if we have one or 200 Buttons
    sprites.update(events)
    # clear the screen
    screen.fill(pygame.Color('white'))
    # draw all sprites/Buttons
    sprites.draw(screen)
    pygame.display.update()
    # limit framerate to 60 FPS
    clock.tick(60)

