When designing for the web you should use RGB colors optimized to be seen on screen.
Illustrator provides different libraries and allows easy switching between CMYK or RGB.
To change a single color, select it, go to the Color menu and select the menu options.
From there, you can easily change it to the color space you require.
To adjust the whole document, go to the File menu and select “Document Color Mode” at the bottom of the menu.