"Red" is a bit of misnomer when it comes to hydrangea color.......none that I would consider a true red. Most are more of a dark cherry pink at best (don't believe online photos - colors never read true).

'Glowing Embers' is known for maintaining its dark pink coloring, regardless of soil chemistry. Another good one is 'Forever and Ever Red', also 'Cardinal'.

Honestly, there hasn't been one that's been tested extensively by people in zone 6 with thoroughly positive results.

I'm still re-siting and waiting on both versions of Forever & Ever Red (Maltisse and Red Sensation) to do something (a.k.a. bloom). You'll get a nice red with F&E Pistachio also, but I've only had mine in one winter and will need to wait and see what it does this summer as well.

Magical Ruby (Kolmaru) is a new one in the US and hasn't really made the rounds yet.

Merritt's Supreme / Oregon Pride can put out some good reddish hues, but need decent winter protection in zone 6. Likewise for Proven Winners' Cityline Paris.

There are also a number of lesser known European cultivars.

Thanks a bunch!