For those of you trying to help, there is a previous thread on this from a few days ago with an image of the hydrangea whole plant:
The thread "Pale and near white leaves on my Nikko Blue..."
in Hydrangeas forum
In both the clematis picture above, and the hydrangea from the previous thread, the paler leaves are not the newest ones. Iron chlorosis can, if severe enough, cause leaves to be almost completely white, a progression from the more usual interveinal chlorosis, but it would be on the newest top leaves whereas this problem is on the lower leaves. Chlorosis starting on lower leaves would be more consistent with nitrogen deficiency but it doesn't usually progress to white.
When a deficiency shows first on the older leaves it is because the nutrient in question is mobile within the plant and therefore when that nutrient is in short supply the plant can move it from the older leaves to the newer ones where it is needed most. When a nutrient is not movable, the symptoms appear first on the newest leaves because the plant can only get it from the soil not other leaves, which is not the case here.
Also, iron chlorosis is usually only a problem when the soil pH is too high which doesn't appear to be the case here either, because the hydrangea is flowering blue and therefore the soil is sufficiently acidic.
Like Greene, I too pondered herbicide injury because that can certainly cause white leaves, but why might it be on the oldest leaves?
This is an odd problem. It doesn't really match typical nutrient deficiency symptoms although the clematis leaves don't look as white as the hydrangea in the pictures. Could it be some unusual combination though, or could N deficiency be severe enough to turn some leaves white? I don't know, but a soil test wouldn't hurt.
Edit: I meant to add, because it may be relevant, that Jason also mentioned daylilies not doing as well as expected: The thread "Daylily growth question..."
in Daylilies forum
That could also indicate a nitrogen deficiency but it's not the only possibility.