Color is subjective, meaning you see it differently under varied lighting conditions. For example, if you are painting an apple, you need to paint the red in the shadow a darker shade from the red highlight. In that sense, you need to overcome what you know (that it is the same red) and paint what you see (a darker color for the shadow) to make the painting look right. A color checker, a simple device that isolates the color so you can see it better, can help. There are commercial varieties available, but you can make a serviceable color checker with things you have at home.
Cut out a piece of card stock 6 inches long and 3 inches wide.
Measure a square that is ½-inch on each side in the middle of the card stock piece, toward one end. You want the hole on one end so you can hold the other end.
Place the card stock on the cardboard and carefully cut out the square with the utility knife.
Hold the color checker in your non-painting hand and point it at your subject. Isolate the area to determine the color as you see it. Move your hand toward you and away to show more or less of the area and help you isolate spots.
Mix colors on your palette and compare them as you look at the subject. You can also dab some color on scrap paper or your palette knife and hold it up to the color checker for comparison.