XPath query for XML node with colon in node name

You need to learn about namespaces and how to define/register a namespace in your XPath engine so that you can then use the associated prefix for names in that registered namespace. There are plenty of questions in the xpath tag asking how to use names that are in a namespace -- with good answers. Search for them.

A very rough answer (ignoring namespaces at all) is:

//*[name()='media:thumbnail']

What worked for me is:

/item/*[local-name()='thumbnail']