This is what I found on the White Flower Farm site. I have to keep my lavender out of the rain as much as possible or they go black.
https://www.whiteflowerfarm.com/how-to- ... creek-grey
Growing Lavender ('Goodwin Creek Grey')
LIGHT: To keep its lovely silver coloring and to produce spikes of dark blue flowers from spring to fall, Lavender 'Goodwin Creek Grey' demands a sunny south window indoors where it will receive 6-8 hours of direct sun. It will also grow well given 14-16 hours of light under a fluorescent light stand.
TEMPERATURE: Daytime temperatures of 60–75°F are fine indoors. In fall and winter, place your Lavender where the nighttime temperature falls to 60°F.
WATERING: Like most silver-leaved plants, Lavender is accustomed to growing in very dry, lean soil. Water only when the top inch of the potting mix is dry to the touch.
FERTILIZER: Fertilize no more frequently than once every 6 weeks during the growing season (April through September) with a balanced fertilizer, such as 20-20-20.
CONTINUING CARE: Lavender 'Goodwin Creek Grey' would be pleased to spend summer outdoors in a sunny location—either in a pot or in the ground. If you live in Zone 7 (0°F) or warmer in the West, you can grow it outdoors year-round. In colder climates, bring the plant back indoors before frost. Lavenders falter in the torrid summers and mild wet winters that are common in the South. If you summer your plant outdoors, keep it in a pot and bring it back indoors for the winter.
Once the flowers have faded on a flower spike, use scissors to cut off the spike close to where it attaches to the stem. Maintain the overall shape of your Lavender by selective pinching of the new shoots. Pinch each shoot between thumb and forefingers; do not shear the plant as though it were a hedge.