4.12.3 ‘Indicate bolded text by changing...’
When the server sends a control sequence indicating that some text should be displayed in bold, PuTTY can handle this in several ways. It can either change the font for a bold version, or use the same font in a brighter colour, or it can do both (brighten the colour and embolden the font). This control lets you choose which.
By default bold is indicated by colour, so non-bold text is displayed in light grey and bold text is displayed in bright white (and similarly in other colours). If you change the setting to ‘The font’ box, bold and non-bold text will be displayed in the same colour, and instead the font will change to indicate the difference. If you select ‘Both’, the font and the colour will both change.
Some applications rely on ‘bold black’ being distinguishable from a black background; if you choose ‘The font’, their text may become invisible.