I've found an easy way to implement this feature without coding that works for me:
- you can enable webforms as a block --> select the webform content you created, tab "Webform" > "Form Settings" > "Enable as a block"
- put the block inside a region, I've put mine in the footer region
- CSS style the footer webform block as "display: none;"
- for colorbox: in "configuration", "Media", "Colorbox" --> advanced settings, activate "Enable Colorbox inline"
- add this as the html code for opening the colorbox webform link:
<a class="colorbox-inline" href="?width=310&height=460&inline=true#name-of-your-block">
