To set the font and colour for the text in the chart key:

chSet 'Kfont' 'he,10,red'  © Helvetica or Arial

The short code for the font is a two-letter mnemonic followed by either or both of the letters ‘b’ and ‘i’ for the bold and italic variants. The font size is in points, and colours use the standard internet names.

See also ...
Colour Cycle
Fonts – defining and using short codes