In all cases, the black texts performed approximately 25% better.  The Brand Book (Style Guide) said we always needed to use white, because it looked friendlier. From a usability perspective, we thought that black would be better, because it is easier to read.  That assumption was right. The Style Guide got adapted. 

The second part of the test was even more spectacular. It was an assumption of a famous Dutch copywriter who claimed that you should almost spell out what your visitors need to do. Despite the fact that it became very text-rich this way, this 'dummy proof ' hyperlink got 28% more clicks than a similar banner with an orange donation button. 

The combination showed a 53% improvement. 

Try hyperlinks more often. Especially in e-mails, you'll see that adding both hyperlinks and buttons increases your total ClickThroughRates