I almost wonder if you'd be better off with a twin or four bulb 2' fluorescent fixture. Fluorescents have a longer history with orchid growers, and it's almost impossible to grow too close to the bulb. Plus, they are nice and diffuse - approximating bright shade closer up.
In my orchid case (modified Ikea) I've tried to get the best of both worlds with the efficiency of LEDs, in that I use a combination of 50W lensless (direct solder) horticultural LEDs and a 40W lensed reflector LED built by Kessil for the aquarium market. In addition to evening out the intensity gradient, I get a nice mix of diffuse and direct light. Most orchids would, I imagine, prefer strong diffuse light, being epiphytes. With a diffuse illumination environment approximating the bright yet still shaded sub canopy environment.
It is almost certainly plausible that orchids will grow under only red and blue narrowband LEDs. (they do use C3 or CAM photosynthetic pathways like most other plants, after all) But will they grow well? This is most likely an unanswered question. I do not know of any studies that look at orchids' response to specific or unbalanced light spectra. It's quite possible that fine tuned spectrum may produce very interesting or useful growth behaviors or flowering characteristics, however the amount of time and expense to conduct such a study, given the long lifespan of orchids, might be prohibitive.
Some background may be found here:
Frontiers | Photosynthetic Physiology of Blue, Green, and Red Light: Light Intensity Effects and Underlying Mechanisms
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