This is often due to color profiles. You take a screenshot (or a mockup), grab the color, and use that value. If you didn't explicitly disable color management

This is often due to color profiles. You take a screenshot (or a mockup), grab the color, and use that value. If you didn't explicitly disable color management beforehand, you've created a new color. Another reason are JPEGs. You get a mockup as JPEG and you use color values from that image. I recommend to put a list of colors at the top of your primary style sheet. That's where others will look for it.