Highly sensitive light sensor to calculate number of times each color LED blinks
Like the OP wrote, you can use very simple sensors, like a phototransistor.
Even simpler, if you can somehow isolate each pair LED-phototransistor, you don't even need to care about the color of the led, just use some bluetac to stick one phototransistor on top of each LED and only measure the brightness.

edit:

A quick check on ebay shows the 3DU5C as good candidate: it has a metallic container, so it is more directional than one with full plastic body.

3DU5C ebay link  and  datasheet

It is shown to have a peak response at 880nm (red light), so you might not be able to use it directly as digital input for the other 2 colors you want to monitor and instead have to treat it as analog reading.

OTOH if you use them all as analog inputs, you will not have to add the resistor that would be required for binary mode.