The field in question has to be defined as 'Rich Text' on the form for you to be able to change the colour, font, size etc (background is anyone's guess though!). The below piece of script should, after declaring rtitem, richstyle, maildoc and session append the text "A new line here" to the field 'body' with the listed font, size, colour and bolding.
Set rtitem = New NotesRichTextItem( maildoc, "Body" )
Set richStyle = session.CreateRichTextStyle
richStyle.NotesFont = FONT_ROMAN
richStyle.FontSize = 10
richStyle.Bold = True
richStyle.NotesColor = COLOR_DARK_BLUE
Call rtitem.AppendStyle(richStyle)
Call rtitem.AddNewLine( 1 )
Call rtitem.AppendText( "A new line here" )
Good Luck!