I don't offer this as an explanation, more a "something to bear in mind". The amount of light held back will depend not only on the colour of the object (few objects are actually monochromatic) but the colour of the light as well. There will be a greater proportion of red light at the beginning and end of the day in the sunlight, and when sunlight is reduced, skylight (from a blue sky) will be blue. Depending on the sensitivity of your meter, you may find an inconsistency, in that red filters (how red is your orange?) can fool meters into underexposing and it's usually suggested adding a stop for a red filter even with TTL metering.
Edit to add:
You really need an Exakta camera with cassette to cassette take up option and a built in cutting knife so you can expose just a few frames at a time.