To set the font and colour for the data value labels:

ch.Set 'Vfont' 'he,8,green'

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 follow the normal internet naming rules.

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