text formatting in report builder

So this is possible by using a little bit of html

If you click on the placeholder (so that the field name is highlighted as below) and go into the placeholder properties

You can then set the placeholder to interpret html (1) 

and then in the Value section (2) you can use an expression like the following

=Replace(Fields!Color_Name.Value,"re", "<span style='color:Red'>re</span>")

I don't have any sample data with "red" as part of the text so I just randomly chose to make the characters "re" red using the following expression

Note you could also build this html version of the text in your data model if it is easier instead of doing the replace in a report builder expression, the important bit is changing the placeholder property to tell it to treat html tags as styles.