There is no standard but consistency is important.
Upper case notation makes sense to separate numbers (decimal or hex) from words for consistency. For example in "1px solid #D0D0D0" all letters and keywords are lowercase (short) and numbers/digits are tall; note that both "0" and "D" are digits.
This is also consistent with the rest of the project which may have different hex syntax like 0xCAD or \xCAD or CADh. Writing 0xcad, \xcad or cadh wouldn't be readable.
