Robot Room’s David Cook shows how an LED and op amp, along with a resistor and cap, can be built into an amplified color sensor.
Unfortunately, even under the best conditions, photodiodes (and reversed LEDs) don’t provide a lot of current flow. The output of the photodiode needs to be amplified for the light-detection signal to be useful in most circuits. A photodiode amplified by a built-in transistor is called a phototransistor.
You can connect a standalone photodiode to the input of a standalone transistor. But, it isn’t easy to control the gain of a single-transistor amplifier, and there are issues with signal noise and the amount of input current required. Instead, a better method for amplifying low-power signals in a high-quality repeatable way is an op amp chip (operational amplifier).