SQL Server 2019

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CREATE TABLE myTable ([ItemID] VARCHAR(3) Primary Key NOT NULL,
	[GroupName] [nchar](50) NULL,
	[Color] [nchar](50) NULL)
INSERT INTO myTable (ItemID, [GroupName], [Color]) VALUES 
('001', 'A', '''blue'''),
('002', 'A', '''blue'''),
('003', 'A', '''blue'''),
('004', 'A', '''blue'''),
('005', 'A', NULL),
('101', 'B', '''red'''),
('102', 'B', '''red'''),
('103', 'B', NULL),
('104', 'B', NULL),
('105', 'B', '''green''')
10 rows affected

WITH temp_id AS (
SELECT max(ItemID) AS MaxId, GroupName
FROM myTable
WHERE Color IS NOT NULL
GROUP BY GroupName
)
SELECT myTable.*, T2.Color
FROM myTable
JOIN temp_id
ON temp_id.groupName = myTable.groupName
JOIN myTable T2
ON T2.ItemID = temp_id.MaxID
;


ItemID
GroupName
Color
Color


001
A
'blue'
'blue'

002
A
'blue'
'blue'

003
A
'blue'
'blue'

004
A
'blue'
'blue'

005
A
null
'blue'

101
B
'red'
'green'

102
B
'red'
'green'

103
B
null
'green'

104
B
null
'green'

105
B
'green'
'green'

ItemID	GroupName	Color	Color
001	A	'blue'	'blue'
002	A	'blue'	'blue'
003	A	'blue'	'blue'
004	A	'blue'	'blue'
005	A	null	'blue'
101	B	'red'	'green'
102	B	'red'	'green'
103	B	null	'green'
104	B	null	'green'
105	B	'green'	'green'