How to tell if baked drumsticks are done?
There is a common misperception about redness in chicken and what it means when gauging the doneness of the chicken. 

Fully cooked chicken can leak juices that are tinged with red, and fully cooked chicken can have pink meat and/or redness in the joints or bones!

From the USDA:


  The pink color in safely cooked chicken may be due to the hemoglobin in tissues which can form a heat-stable color. Smoking or grilling may also cause this reaction, which occurs more in young birds.


The acidity in the tissues can case a similar effect in the myoglobin.

From an unnamed research assistant at a major chicken processor speaking to Meathead Goldwyn at AmazingRibs.com:


  "When the muscle is high in pH [low in acid] it takes a much higher temperature to denature the myoglobin. The meat may need to be 170 to 180°F before the myoglobin in breasts is denatured sufficiently to see clear juices. The drumstick and thigh have higher levels of myoglobin and they require an even higher internal temperature to denature it. 


Red or purple bones and adjacent meat are caused by modern chicken manufacturing that can produce chickens big enough to slaughter at an age much younger than used to be the norm:


  Darkening around bones occurs primarily in young broiler-fryers. Since their bones have not calcified completely, pigment from the bone marrow can seep through the porous bones. Freezing can also contribute to this seepage. When the chicken is cooked, the pigment turns dark. It's perfectly safe to eat chicken meat that turns dark during cooking.


-USDA

Furthermore, chicken that has not reached a safe temperature can have meat that looks done and juices that run clear!


  If the muscle pH is low then the myoglobin is denatured at a lower cooked temperature. This means that one might potentially see clear juices at 150 to 160°F and this is not safe.


-AmazingRibs.com

According to the same source, the PH of the muscle tissue can be affected by the chicken's genetics or pre-slaughter stress. That and the fact the source doesn't want to be named causes unpleasant images to appear in my mind. But I digress...

In my experience, dark meat is best cooked to the point that it has begun to shrink away from the bone; which is way past the point of it having reached a safe temperature. Therefore laxity is better test of doneness than color for dark meat. Another way to describe that is that the joints between the thigh and the body and the thigh and the drumstick move freely.

To be considered safe, chicken must be cooked to or above 165 °F (73.9 °C). Chicken breasts are (IMHO) best right at that temperature. Getting good at hitting that temperature takes some practice.  White meat is much more sensitive than dark meat to overcooking, so testing frequently with an accurate thermometer is really the only way to go until you have the considerable experience necessary to accurately gauge the meat's doneness without the thermometer.

Drumsticks are very unlikely to be undercooked after having been baked at 425°F (218°C) for 40 minutes. Watch for meat that has begun to shrink from the bone. You should be at or beyond that point already, which as I have shown, is a far better indicator of doneness than color. Of course, an accurate thermometer is always your safest bet.