'High Key' refers to pushing all the values (darks and lights) towards the lights. It means the darkest dark in the painting is no darker than a middle grey.
Here's a recent painting I completed in this vain:
"Beating the Heat (in Paradise)", 12x16, Oil
Painted during the Carmel Art Festival
For comparison purposes, here's a photoshop'ed version of the painting with he values stretched out of the whole white-to-dark spectrum (a more conventional way of painting):
Photoshop'ed version - using a full spectrum of values from black to white
Here are greyscale versions of both. In these versions, you can really see the values:
Top - original painting in greyscale
Bottom - photoshop'ed to use full range of values in greyscale
Interestingly, it's possible that the full-value-range version reads better in black-and-white. Why is this? Because color becomes more essential when there is less range in the values. But certainly when viewed in color I feel that the high key version gives the painting a different sense of life and vibration. It simulates the experience of glare and heat, and makes for an interesting painting.
Doing a painting in high-key is a fairly delicate operation. One needs to keep the relationship between all the values true, but since each value is so much closer to the others it takes a fine balancing act to mix each color note accurately. The finest mistake in a value choice can throw the whole painting off. To illustrate this, I used photoshop to darken a few of the darks. You can see it throws the whole painting into a different direction:
Photoshop'ed with key darks made darker.
This throws the painting off ever so slightly.
Can you spot the darker marks?
High Key painting is about subtlety. Remember that subtlety doesn't always translate well into photographs and online viewing. The original high key version really sings when viewed in person. As a testimate to this, the painting got multiple bids at auction during the Carmel Art Festival and was a crowd favorate judging by the number of people who came to find me to comment on the painting. It was fun to do and a lot of fun to show.
To explore this further I recently did a larger studio version of this subject - and I've very pleased with the result. Check it out here.