Quality & Freshness
In fact, the color of matcha is one of the clearest ways to determine how fresh it is, and also of what level of quality it is. What you are looking for in matcha is always that healthy, deep green. The more of that you see, the better quality it is going to be, and the fresher it probably is too. When it is of poorer quality, from an inferior brand, or it has been left out too long, it can fade to a pale green.
Then, if it is left even longer, it can even become a kind of yellow. This is a sign that much of the goodness has been stripped out of it, and you are not going to get the same matcha health benefits from it - nor the same delicious taste that you have come to expect.