you can make your plant sweet by adding sugar, but it needs to be in a form the roots can absorb like molasses. blackstrap molasses works better 1-2tsp per gallon
if purple is not hereditary to the strain it could be a sign of stress. Its true some strains will turn purple if they have cold nights during the last few weeks of flower. due to cold weather stress. also a plant turning purple could just be because of its genetics, even if it hasn't been stressed at all
can buy them online..
if you want a vanilla flavored smoke, why not by like vanilla kush or another vanilla flavored strain to start with.. barneys farm sells vanilla kush i do believe.. check out the attitude, located at the top of the screen under the seeds link..
Any decent smoking store sells them, you only need 2 drops or so so they last ages. Just depends if you like it or not but for about £2 its not exactly a big waste if you dont like it
Food coloring and vanilla extract? C'mon man, get the right strain and it will taste and look great without screwing up the bud. You going to start watering with orange juice to make it taste like oranges too? How about spray some of that fake snow they sell at Christmas onto your buds to make them more frosty?