Automatic transmission fluid, or ATF, sometimes changes color. While the fluid may still be fine, a change is color is typically not a good sign and should be investigated right away. Usually what happens is the fluid gets overheated and breaks down. This can cause more overheating and more fluid breakdown, a catch-22. The more overheated the fluid gets, the quicker it breaks down. This is the most common cause of transmission failure.
Remove the transmission dipstick. Check the ATF color on the dipstick and its smell.
Realize first that ATF fluid now comes in different colors. It used to be a clear reddish color, but now that is just one of the colors used. If you just had the fluid replaced and the color has changed, the repair shop may have used a brand that is a different color. But you may want to verify this with the shop.
Make sure the fluid is clean and has no odor. Normal ATF will have a color like red, green or blue, but should otherwise be clear. This is good transmission fluid. Check the fluid color regularly to note any changes.
Keep a close eye on the ATF if it's a little darker than normal or has a light smell. This can happen to good ATF, so a slight change may be okay. On the other hand, the ATF could be in the early stages of breaking down. Check it frequently until you determine which is the case.
Replace the fluid immediately if it's more than a little darkened and has more than a little burnt smell. This indicates the fluid has broken down to some degree. The darker the fluid and the worse the smell, the more broken down the ATF has become. If it's nearly black with a very foul burnt smell, your transmission may already be ruined; take it to a mechanic.
Check for coolant leaking into the ATF. This is indicated by a milky/cloudy color, which means an internal seal is leaking, allowing coolant to leak into the transmission. This should be fixed immediately.
