When she/he arrives at a point where the colour changes, the knitter brings the new colour up underneath the old one (to prevent holes) and starts knitting with it.
What with spring bulbs giving way to early flowering perennials and shrubs, these then overtaken by summer bedding, you can have a riot of colour up to mid-summer.
To choose the right colour hold the colour up to your face, and if it automatically makes you look pale, opt for a different shade or colour completely.