In "Awareness Itself", we read the advice given by Ajaan Fuang, as recorded by his student Thanissaro Bhikkhu:
§ A young nurse practiced meditation with Ajaan Fuang several days
running, and finally asked him one day, "Why wasn't today's session as
good as yesterday's?"
He answered: "Meditating is like wearing clothes. Today you wear
white, tomorrow red, yellow, blue, whatever. You have to keep
changing. You can't wear the same set of clothes all the time. So
whatever color you're wearing, just be aware of it. Don't get
depressed or excited about it."
§ A few months later the same nurse was sitting in meditation when a
sense of peace and clarity in her mind became so intense that she felt
she would never have a bad mood infiltrate her mind again. But sure
enough, bad moods eventually came back as before. When she mentioned
this to Ajaan Fuang, he said, "Looking after the mind is like raising
a child. There will have to be bad days along with the good. If you
want only the good, you're in for trouble. So you have to play
neutral: Don't fall in with the good or the bad."
§ "When the meditation goes well, don't get excited. When it doesn't
go well, don't get depressed. Simply be observant to see why it's
good, why it's bad. If you can be observant like this, it won't be
long before your meditation becomes a skill."
§ A student came to complain to Ajaan Fuang that she had been
meditating for years, and still hadn't gotten anything out of it. His
immediate response: "You don't meditate to 'get' anything. You
meditate to let go."
§ "You should watch visions the same way you watch TV: Just watch it,
without getting pulled inside the tube."