For one thing Osram labels their bulbs as RGBW with distinct mention of white color temp from 2,000-6,500K. That might mean that you can send the bulb a color temperature command and it will switch to “white” mode to give the range of white including warm white. If you’re a tinkerer, you could copy over all the colorTemp code from the Tunable White devicetype. Then you’d have the color picker and a separate “Color Temp” slider.
Try this device type. No idea if it will work, but using the new color temp slider might bring the bulb back to a range of white. So after you’re on “red” or something, try sliding the color temp piece to 2700.