The Von Restorff Effect, which is also sometimes known as The Isolation Effect, predicts that when multiple similar objects are presented, the one that differs most from the rest is the one most likely to be remembered.
The theory was developed by German psychiatrist Hedwig von Restorff, who, in her 1933 study, found that when participants were presented with a list of categorically similar items with one distinctive, isolated item on the list, memory for the item was improved.
The most basic application of this effect is that whenever we are designing anything we should look to make the most important information, or most important action, visually distinctive from other things on the page.
This is why we find pages on websites with grey buttons apart from the green "proceed" button,
The counter to this is that we should look to use restraint when placing emphasis on visual elements in order to avoid them competing with one another.
It should be stated that we shouldn't just rely on colour to make these distinctions, as we still need to cater for people whose colour recognition is different.