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13- Invisible Text, Unknown consequences
Invisible text is the practice of having the text in the same color as the background, so that the visitor does not see this text but the search engine will think it is part of the website.

While no one can verify if such methods are being penalized (i used to use it and it worked always), You should not be taking the risk, search engines are ongoing developed, and once (if not already) they have an algorithm to detect and a policy to penalize this practice, your website may be black listed and never coming back again (unless you change its domain and hosting (domain banned or IP (host) banned). I really advise you not to use such a method