Yes, it’s safe to paint a glass ball floor lamp if you do it correctly:
Remove the glass lampshade from the lamp, wash it using warm soapy water, and dry it with a paper towel
After wiping it down with some rubbing alcohol, try and only handle your glass lampshade from the inside
Choose the right glass paint, not stained-glass filler (if you want a brighter effect, give it a translucent finish; for a sea-glass look, opt for a matte finish)
Apply it with a flat brush, using brushstrokes that all point in the same direction
Let it dry for 24 h