Your divorce went smoother than you thought it would. You and your wife managed to come to a civil agreement on everything, without resorting to litigation. Your children seem to have coped okay too. The child custody arrangement where they live with their mom during the week and spend the weekends with you has worked out well.
You and the kids spend the weekends hiking and camping in the mountains around Boulder. You have some great conversations around the fire and feel closer to them than ever. Then your wife rings to say she has a job offer in California and wants to take the kids with her. That would put an end to your weekends with the kids.
If you agree to the move, the two of you can ask the court to modify the child custody order. If you do not agree, she can file herself, but it will be up to the judge to listen to the reasons for and against and decide if it is in the best interests of your children.
A judge is unlikely to grant permission if it means significantly reducing the time you spend with your children, which it would in this case. If you were a lousy parent, had a poor relationship with your children or were a danger to them, a judge may be more likely to give your ex-wife permission to move state with the children.
If you need to modify your Boulder child custody arrangements for any reason, seek legal help to understand the options available.