As of now Salesforce doesn't support conditional formatting of field values even for formula fields. This is still in Idea stage https://success.salesforce.com/ideaView?id=08730000000BrPbAAK.
Only rich text fields support formatting. Here is a Success Commuity Link similar to the requirement where a custom rich text area field is used instead of a formula field and the field value is updated with HTML formatting using a workflow field update.
First Create a Rich Text Area and then Create a Field update as below with a similar formula to update this Rich Text Area. Now fire this field update with a workflow rule.
IF(Amount < 100000 ,('<html><body><div style="color: red;"><b>' & TEXT(Amount) & '</b></div></body></html>'),
IF(Amount > 100000 ,('<html><body><div style="color: green;"><b>' & TEXT(Amount) & '</b></div></body></html>'),