From MetroAmp Wiki
Given the nick-name "Mustard" caps due to their greenish-yellow color, these capacitors were actually manufactured in the UK by Philips or Mullard. They are ubiquitous in English amps from the 60's and 70's. Including Hiwatt, Vox, Marshall, Sound City, Watkins and more.
Technically speaking, these caps are polyester foil, self healing construction. Available in 160V or 400V capacities and a range of values. See original Philips data sheet:
See also: interpreting mustard cap date codes Dating mustard caps.
Some examples:
