1/2 teaspoon is around 50 drops. You need to cut back on your amount of lemon juice.
I recommend diluting the mixed solution with water. That way you can tell if it is too high a concentration. Jim Humble refers to the diluted solution having around 1 PPM free chlorine dioxide. The color of a 1 PPM free chlorine dioxide solution is nearly Crystal C.. If your solution is yellow, the concentration is over 1 PPM.
I believe your solution turned a yellow because of the 2 teaspoons of lemon juice you had in it. I just mixed a glass of water with 2 teaspoons of lemon juice and it had a pale yellow color to it. Chlorine dioxide yellow is a deeper yellow.