I've never really thought much of it but now that you mention it I just always thought they tasted bad. I never took the time to actually think what they were. I don't really feel the need to think farther because your flavors seem fitting enough! They make a lot of sense.
I honestly think they would taste pretty bitter but really who cares what it taste like. It heals all your hearts and other effects which is fine by me.
Red- Like Iron, and bitter
Green- Like grass (Yes, i've eaten grass)
Blue- Like water, but with a very small hint of berries
Guardian- Old milk
Shield- Cough medicine