Hi,
hi I'm trying to make the difference between two colors red and blue without using a color sensor
If you use two LDRs (or phototransistors) with a filter of each color (gel filter or just try some translucent ("clear") colored plastic), you should be able to differentiate them.
You may need to add some light, depending on whether the source of these colors are colored light itself, or just colored objects reflecting ambient light.