Nova Zembla is bright red. It is the most popular red and does well in hot climates and in cold climates.

There are several dark, dark reds. They include:

Nova Zembla: Flower bright red with black spots.

Francesca: carmine red with huge flowers.

Besse Howells: deep purplish-red with a deep red blotch.

Mrs. P. den Ouden: strong purplish red, with a ray of olive green markings in the throat.

Sumatra: deep red shading to strong red in center.

War Dance: strong red with black blotch and spotting.

A couple more tender dark reds are:

Carmen: waxy, dark red with fine, slightly deeper colored veins.

Jean Marie de Montague: strong red with dark red spotting in the dorsal throat. Probably the most blood red rhododendron.

Taurus: vivid and strong red with darker throat and black spots on dorsal lobe.

I have Home Depot special called 'Firestorm' (I think) which looked on the plant label to be Red, but when it bloomed, it was a very dark red -- I would call it Garnet, or possibly maroon.