That’s not a new column, that’s a new DataFrame:
In [11]: df.groupby(["item", "color"]).count()
Out[11]:
id
item color
car black 2
truck blue 1
red 2
To get the result you want is to use reset_index
:
In [12]: df.groupby(["item", "color"])["id"].count().reset_index(name="count")
Out[12]:
item color count
0 car black 2
1 truck blue 1
2 truck red 2
To get a “new column” you could use transform:
In [13]: df.groupby(["item", "color"])["id"].transform("count")
Out[13]:
0 2
1 2
2 2
3 1
4 2
dtype: int64
I recommend reading the split-apply-combine section of the docs.