About 90% of what you perceive as taste actually comes through your sense of smell.
When you have cold, your sinuses are likely to be filled and the air with the aroma can't get past the sinus. This deprives you the sense of smell.
So most of your sense of taste is gone along with your sense of smell.
An onion, apple and potato all have the same taste. The differences in flavor are caused by their smell.