When you reply to a message, the icon of the message in the message list changes, the left down corner of the envelope changes colour.
As a sidenote, when you forward a message, then the right down corner of the envelope changes colour.

Yes, that is possible. For this you need to do the following:
Create a colour group
(Message -> Colour groups -> Edit colour groups)
Set the font properties to a font and colour of your liking.
Next you create a Replied messages filter. Set the condition of the filter to 'all messages' and set the action to 'Set message colour'
This will set the colour of all messages that you will reply to to the colour group you assigned in the filter.

Unfortunately this will not work for messages you already did reply to. However, when you click in the message list on the envelope on top of the column with envelopes (I'm explaining this rather clumsily, but I don't know how to to it better) you will sort the messages on their status (unread, read, replied, etc) this means that all replied messages are sworted together, therefore making it easy to select them and assign your colour group in one sweep (per folder).

When you've got a lot of folders containig replied messages you can select them in a virtual folder:
Folder -> New virtual folder -> 'Filter' tab -> Advanced filter settings -> Message (attributes) - is - replied
And select all folders as source for this virtual folder. Now you'll have all messages you replied to, select them all and assign your colour group.
When you're done, you can delete your virtual folder again.