For digitizing slides, the safest goal is consistency and a neutral capture, not trying to guess an “intended” projector color exactly.

From the answers: traditional slide projectors used tungsten/halogen lamps around 3200–3350K, and slide copy film was commonly balanced for Tungsten A (3200K). So if you want to mimic projector viewing, a stable ~3200K light source with a fixed tungsten/halogen white balance is a reasonable approach.

That said, some film/viewing standards for transparencies use D50/5000K viewing tables, and slide film was often valued for its particular color rendering rather than strict accuracy. In practice, using daylight can vary too much unless you can keep it consistent.

Best practice: use a stable, high-quality backlight, keep the camera white balance fixed, and avoid auto white balance. Measuring your diffuser/backlight can help with consistency, but changing natural light makes it a poor reference. If possible, use a dedicated, repeatable light source instead of daylight.

So: for projector-like results, use about 3200K; for archival consistency, use a stable calibrated light source and fixed WB.