in keeping with my curmudgeonly advice

[waider]

Found on MetaFilter, the correct answer to "how do I draw a shamrock on a head of Guinness?" is "Don't. It's a drink, not a canvas. " or words to that effect. Although props to the barman at Mr. Pickwick in Biel who, when told I didn't care for a shamrock on my pint, drew a pair of breasts instead.