Relax and enjoy it!

i wouldnt worry about cool colors, warm colors and clashing of colors... mother nature doesnt so neither do i... mix and match - cluster and clash - a wide variations in colors and textures make an excellent display... of course thats just me and probably not the "garden guru" of reccomendations... you need to stick with what you feel comfortable with - you dont want to plant it and then every time you look at it think " eeiiww that clashes" ...

I have cool colors in my shade garden too. Hot colors don't quite look right to me but that's more of an opinion that anything else. I also have a heuchera in there that blooms red BUT it's not that noticeable. The flowers are small and airy so it sort of blends in. Red doesn't show up as much in the shade as it would in full sun. If you want more red to be consistant, I'd just divide the heuchera (when it's big enough) and plant some more of it closeby.

You could try the Japanese painted ferns, 'Burgundy Lace' has red stems and there's red in the leaves, which would tie in with the red theme. 'Ghost' is a beautiful, more upright cultivar with grey-white foliage, a good contrast to red and matches your cool colors.

How about variegated Solomon's seal? It has green leaves with a little white, but the stems are a maroonish red. Ligularia Desdemona has reddish brown undersides to all it's leaves. Geranium samobar has green leaves with maroon markings. There are all kinds of ways to put little touches of red through the border so your heuchera doesn't look so lonely. None of the plants I can think of scream red so the effect will be subtle.

Primula japonica has a nice deep red cultivar. Wild Canadian columbine--red with a bit of yellow. Lobelia cardinalis--red,but likes it moist--mmay be difficult to provide adeaquate moisture. Too, buy a few more 'Ruby Bells' ao the one doesn't appear isolated.