This winter I built a small indoor seedling greenhouse with high-power LED lighting (24/7), constant air movement, and ventilation with temperature and humidity setpoints of ~70°F and ~70% RH.

I started two batches of seeds (Thai Hot Pepper, Basil, Catnip and Cilantro) in no-dampoff mix, one inside the greenhouse and one in a south-facing windowsill.

The seedlings have sprouted and have grown a few inches. All the seedlings grown in the greenhouse, except cilantro, are about 2x the side of those from the windowsill. However, while the windowsill seedlings are a uniform vivid green, the greenhouse seedlings are more pale. Many of the greenhouse seedlings' first (non-true) leaves have dried up. The cilantro in the windowsill has grown much more, which I'm guessing is due to lower temperatures.

The LEDs are 75% red and 25% blue and very high power; the greenhouse seedlings are not leggy and have larger and more leaves than the windowsill ones.

I have searched for an explanation for the appearance of the greenhouse seedlings, but failed. I'm wondering if the 24/7 lighting, lighting spectrum, light intensity, high humidity, or lack of nutrients (due to accelerated growth) could explain why my greenhouse seedlings look less healthy and green yet are growing more foliage? Thanks in advance for any insight.

I'm guessing the greenhouse seedlings have the excelerated growth because of the optimum lighting. They shouldn't be leggy; seedlings grown on windowsills are usually leggy because of the limited light. You did not indicate that you are following a fertility regime, if any. It would be a fair assumption that the increased growth is putting a drain on the plant for nutrients. I don's set any other explanation. I think bumping up and/or starting a fertilizer program will help.

As my original question was from some time ago I have been able to do several more sets of seedlings since. The one parameter I've varied that made an obvious difference was increasing the ventilation. My original greenhouse was nearly a sealed container (apart from maintaining the high humidity) so I suspect it must have been lack of fresh air. If I recall I was using a dilute general purpose fertilizer for both sets of plants.

Interesting that ventilation would make a difference in color. Forgot to mention that they should be fertilized about every two weeks.