Unrefined sugar has much more flavour; do try to use golden caster sugar instead of white (except for snowy white meringues). Muscovado sugars are quite delicious; their taste and colour come from the molasses clinging to the grains. Light muscovado has a butterscotch flavour and creams very well. Dark muscovado requires a little more care - it is a strong flavour that could easily overpower - but its delicious treacly taste is perfect in rich fruit cakes. If it goes rock hard (as it will if the packet is left open to the air), empty it into an ovenproof dish, cover with foil and soften in a very low oven.