I'm not the salvia guru by any means but my darkest red is Greggii Flame.

Flame is a good red S.greggii,Salvia greggii "Red Star" from colorspot nurseries is a larger flowered form the plant gets bigger also 4' x 4'. I know most of our red
Salvia are typically Orange red. The Salvia greggii are
sun plants 6hours of direct sun is full sun.
If you live in wetter areas S,microphylla would be a better choice. A Shade loving Salvia for the Austin/San Antonio,Tx. Hill country area would be S.roemeriana
aka Cedar Sage. What part of Texas are you in?
Art

For true and pure reds, Furman's Red, Keter's Red (nearly identical, as far as I remember) for greggiis: Maraschino, Cherry Chief, and Cherry Queen for greggii hybrids; and deltoid-leaved, Hot Lips, Orange Door, Zaragoza, Red Velvet for microphyllas (the last a greggii hybrid?)

These colors are as dark as you can make a true red without adding black and turning it into a kind of brown. On an RGB (Red, Green, Blue) scale of 0 to 255, the R is at a maximum of 255, and the others at a minimum of 0. Of all these sages, I can't discern any deviations from the flowers when I scan them on my flat bed and measure them using Photoshop tools. Any perceived differences are usually caused by variations in lighting, background, and tweaaks to the color caused by optical (diffraction) effects of the fine hairs on the flowers that actually carry the pigment.

Thanks for your suggestions. We've moved to a new hosue and I'm starting a hummingbird garden. At my old house I had several different salvias, but one in particular was a deep, pure red and I forgot its name. I don't think it was Furman's (mine were red with a tiny hint of pinkish). It might have been "flame". I'll try that one and a few others that y'all have suggested.

I'm in the North Texas area so I guess I'm zone 8.