alucinar

Literally to hallucinate, but in everyday colloquial Spanish (especially in Spain) it's used constantly to mean to be amazed or astonished — no doy crédito, no puedo creerlo. Alucinar en colores intensifies it further, meaning to be utterly floored by something.

• Alucino con lo rápido que ha crecido tu hijo. — I can't believe how fast your son has grown.
• Bajo los efectos de la fiebre, empezó a alucinar. — Under the effects of the fever, he started to hallucinate.
• Vas a alucinar cuando veas la casa nueva. — You're going to be amazed when you see the new house.

Assuming alucinar always means clinical hallucination — in casual conversation it's overwhelmingly used as hyperbole for surprise, not a claim that someone is literally seeing things.

• expression "Vas a alucinar" — A widely used colloquial expression in Spain meaning "you're going to be blown away/amazed," one of the most common everyday uses of alucinar outside its literal clinical sense.