You can simply use raw html tags like
foo <font color="red">bar</font> foo
Be aware that this will not survive a conversion of the notebook to latex.
As there are some complaints about the deprecation of the proposed solution. They are totally valid and Scott has already answered the question with a more recent, i.e. CSS based approach.
Nevertheless, this answer shows some general approach to use html tags within IPython to style markdown cell content beyond the available pure markdown capabilities.