How to keep highlights bright in a coloured pencil drawing.
Planning your colours for a coloured pencil drawing and how you will build up layers at the beginning of your drawing is key to keeping your colours fresh and the highlights bright.
Make a test selection of colours for your drawing to make sure they will work together in a harmonious colour scheme from the lightest colours to the darkest.
Make a note of where your highlights are and keep these clear of colour as you establish more of your values, colours and form of the subject.
If you are using smooth white paper you will not be able to layer light colours over dark, this can only be done if you are using a sanded paper or Pastelmat, so preserve your highlights from the beginning and remember that you will only require light pencil pressure over these areas as you can use the white of the paper to lighten your colours.
Just as importantly, having a dark colour laid down in the correct place early on allows you to establish your value range in the beginning stages and helps you judge the depth of your colours throughout the rendering of the drawing.
Work around the highlight areas with soft edges especially if the form is round, such as an eye or as in the muscle structure on this horse, then build up your depth of colour around the highlights in many light layers. Gently adjust your highlights toward the end of the drawing keeping your lights light and darks dark.
I hope you have found this tip helpful as this question is asked quite often.