I'd like to put the white object repeated along the path, but with Path -> Path effects all the clones/copies become grey (no gradient), and with LPE it just copies the stroke, no fill. How could I do it?

By converting the object to a marker.
Similar to this.

bartovan wrote:I'd like to put the white object repeated along the path, (…)

With 'Extensions > Generate from Path > Scatter' maybe?

~suv wrote:With 'Extensions > Generate from Path > Scatter maybe?

That definitely does the job perfectly, great, thanks!