The key in Diggs et al. 1999, Illustrated flora of North Central Texas (BRIT, p. 758) gives color distinctions for the H. drummondii complex, repeating R.S. Irving's 1980 features but adds corolla color. So far as I can determine color, even leaf color, does not distinguish these taxa. Neither fresh materials nor dried specimens support color as a reliable characteristic.
The following table gives the colors from Diggs et al.
|Leaf Color||Corolla Color|
|H. drummondii||bright green||blue|
|H. reverchonii var. reverchonii||gray green||white or
|H. reverchonii var. serpyllifolia||dark green|
Although online photos as well as my own show a wide range of colors for all three taxa, with rare exceptions the corolla color is a shade of lavender (lavender-purple to light pink-lavender). Blue flowered H. drummondii may exist, but I would need to see it in the field or see a photo in full sun to convince me (since lavender tends to photograph blue in shade). I have not seen white corollas, but in strong sunlight the pale lavender often appears to be white.
The following 3 photos illustrate less common colors, in part due to lighting conditions. The center image is in the shade. The rightmost image was taken in early morning, and might appear white in bright sun. Place the cursor over an image to get the ID; click for enlargement.
The images below with more 'normal' color illustrate the difficulty in using color.
|H. drummondii||H. reverchonii var. serpyllifolia||H. drummondii||H. r. var. serpyllifolia||H. r. var. reverchonii|
|Hays Co., May 12, 2012||Travis Co., May 12, 2012|
|H. drummondii, Travis Co.|
|H. reverchonii var. serpyllifolia, Travis Co.|
|H. reverchonii var. reverchonii, Travis Co.|