Давайте возьмем ваш код построчно:
Код 1:
lst= ['Red', 'Green', 'White','Black', 'Pink', 'Yellow']
del lst[0] # lst = ['Green', 'White','Black', 'Pink', 'Yellow']
del lst[4] # lst = ['Green', 'White','Black', 'Pink']
del lst[5] # error as the max index remaining in the updated list is 3
print(lst)
Код 2
lst= ['Red', 'Green', 'White','Black', 'Pink', 'Yellow']
del lst[0] # lst = ['Green', 'White','Black', 'Pink', 'Yellow']
del lst[4] # lst = ['Green', 'White','Black', 'Pink']
del lst[3] # lst = ['Green', 'White','Black']
print(lst)