Beat egg white with mixer until fluffy, mix icing sugar with corn starch & add in bit by bit while mixing, add in the flavor & mix well until well incorporated then add in the vinegar or lime to further thicken it divide the paste in to 2 & add food color to one then decorate ur cake with it.

Note-if you dont have icing sugar at hand you can blend ordinary sugar & corn starch until powdery then seive it & use it. The essence of the corn starch is to further thicken it like wise the vineger & lemon. To make the decoration more neat so it wont be too watery. U can google the exact measurement of 250g icing sugar in cups using normal sugar.