Brake fluid and power steering fluid are not interchangeable and should never be mixed or substituted for each other. They are made of different substances and have different functions.
Brake fluid and power steering fluid are both clear, though brake fluid may have an amber cast. Power steering fluid may be pinkish to red as well.
Brake fluid is designed to carry the force of the driver's foot through the master cylinder to the calipers, which apply the force to stop the car. Power steering fluid transmits hydraulic pressure to make steering easy.
Brake fluid is designed to have a high boiling point and to not be compressible. Power steering fluid has to function in any temperature and provide lubrication to the pump and valve system while maintaining the rubber components and minimizing noise.
Brake fluid should be changed when it gets dark and cloudy. Some manufacturers recommend changing it every 30,000 miles. Check your owner's manual for the recommended interval for your car. When to change the power steering fluid varies by car and power steering fluid type. Some can go 100,000 miles between changes, some recommend a change every 3 years or 50,000 miles. Again, check your owner's manual for the recommendation.
A great deal of harm can be done if you confuse brake fluid and power steering fluid. In the case of putting either fluid in the wrong system the seals will be compromised, which will result in extensive, costly repairs. It is best to pay close attention to which fluid you are putting in which system and avoid the situation completely.
