Assuming you fix your naming convention issue (yes - SQL Server will pick up on the invalid name issue), you can use a query like the one below (i fixed the names to make this work)

declare @x xml

set @x='<PartsList>

<a123>

<Color>blue</Color>

<Size>small</Size>

</a123>

<a124>

<Color>black</Color>

<Size>medium</Size>

</a124>

<a125>

<Color>red</Color>

<Size>large</Size>

</a125>

</PartsList>'

select c.value('local-name(.)','varchar(100)') colname,

c.value('(./Color)[1]','varchar(100)'),

c.value('(./Size)[1]','varchar(100)') size

from @x.nodes('/PartsList/*') vm(c)

EDIT: sry - I am looking at the wrong end of the issue - you're attempting to generate this - not parse it.

I am pretty sure this is possible using reasonably advanced FLOQWR statements, but it may take some time to get this right.