Is God invisible and is God a Spirit?

Yes. The triune Godhead is invisible and is Spirit. The verse calls God the Son the image of the invisible God. At the appropriate time, one member of the Godhead took on visibility, becoming the visible manifestation of the invisible God.

Les explains that the verse is speaking of God the Son, who is the image of the invisible God. The Spirit God, the triune God, the Godhead, is invisible and is Spirit. But at the appropriate time, one member of the Godhead took on visibility; God the Son became the image, or visible manifestation, of the invisible God, the firstborn of every creature. Les also connects this to creation: by God the Son all things were created, visible and invisible, whether thrones, dominions, principalities, or powers—all created by Him and for Him. He is before all things, from eternity past; He did not begin at Bethlehem but has always been, and He holds all things together. He is also the head of the body, the Church.

Colossians 1:15

Who is the image of the invisible God, the firstborn of every creature:

Who is the invisible God referred to in Colossians 1:15?

The invisible God in Colossians 1:15 is the triune God—Father, Son, and Spirit—who are Spirit and therefore invisible. The Son, referred to by the pronoun 'who,' is the visible, tangible image of that invisible God, having stepped out from the triune God and become visible, especially in the flesh at Bethlehem.