At the end of December my company stock was cashed in due to the sale of the company. This was a long term asset. I know that I need to pay 15% capital gains tax on that - is there also a State Capital gains tax?

Yes - Colorado has a flat tax of 4.63% that is based on your Adjusted Gross Income as reported on your federal tax return (with some additions and subtractions). Since your AGI includes capital gains and losses, they are included in CO's income tax.