Unfortunately, at this time it's impossible to partially format anything that is a result of a formula.
There is a question (and answer) on Stack Exchange about this:
https://webapps.stackexchange.com/questions/87710/google-spreadsheet-partial-formatting-using-formula-in-cell
Answer stated:
Partial formatting of cell content is a recent addition to Google Sheets, and it not really integrated with the rest of spreadsheet logic. The only thing you can format in this way is a string entered directly into a cell, as some text here. Using any kind of formula, even the simplest ="some text here", is incompatible with partial formatting.