Value is the degree of lightness or darkness of color and certain colors can fool the eye. For instance, when I see a red rose surrounded by green leaves of the same value, I usually think the red looks brighter.
Maybe because it’s on a bright screen and not in nature, the green looks brightest in the example below. The rectangle underneath is those colors blocks converted to gray scale.
Here’s a good exercise for learning to see color as value. Make a solid hatch at the edge of a piece of paper and then hold the paper up, squint your eyes, and find a color that matches it in value. It’s important to make the swatch on the edge of the paper so there won’t be any white showing between it and the color.
This is great warm-up exercise too. Here’s another variation.
Compare the hatch to any color, highlight or shadow, and ask yourself if it’s lighter, darker, or a match. Just a minute or so of this will help you get into your drawing frame of mind.