I am continually vexed by color: its opacity, density, translucency. Blues are the most persnickety, greens are the most agreeable. Red is a constant mystery. Is it yellow I want, or is it gold? And what happens with any of these when I paint it onto sandblasted glass, which possesses its own translucent/opaque persona? Zinc white adds depth, but refuses to blend properly.
And light vs. shadow, direct-under-the-lamp light vs. natural light: every shift of light alters what it is I see when I'm attempting to paint a piece of glass.
And, of course, there is my own particular mood.
And the depth of the blast on the glass, where a millimeter variation can greatly influence paint application.
And the grit in the sandblaster: fresh and new? Or old and spent?
There are bad paint days, especially bad black paint days, usually at the week's end, when it's time to knock off and let my weary fingers/hands/arms/shoulders take a rest.
Patience, I tell myself.
Love the glass.
Love the paint.
(Follow my own advice.)