Replicate rows along all month from the first day

If you do not have duplicate rows with the same DATE then you can skip the Table.Group step.  The Grouping is to avoid generating the same list multiple times which can slow down the query due to unnecessary repetition.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjDUByIjAyNjJR2lENdgH0cg7eTj6OytFKsDkTaCSfuGBoc4+rmDFYS6KsXGAgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [DATE = _t, CAR = _t, COLOR = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"DATE", type date}, {"CAR", type text}, {"COLOR", type text}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"DATE"}, {{"Count", each _, type table [DATE=nullable date, CAR=nullable text, COLOR=nullable text]}}),
    generate_dates = 
        Table.TransformColumns(
            #"Grouped Rows", 
                {
                    "DATE", 
                    each List.Dates(
                        _, 
                        Duration.TotalDays(Date.EndOfMonth(_) - _) + 1, 
                        #duration(1,0,0,0)
                    )
                }),
    #"Expanded DATE" = Table.ExpandListColumn(generate_dates, "DATE"),
    #"Expanded Count" = Table.ExpandTableColumn(#"Expanded DATE", "Count", {"CAR", "COLOR"}, {"CAR", "COLOR"})
in
    #"Expanded Count"