I think your manager is right, and that this phenomenon is much more widespread than just photos for social media, or even photography in general.
Take a look in supermarkets, and take in the flashy bright colours and stark contrasts brands use to attract your attention. If they get you to look at their product, chances of you buying it shoot up dramatically. That's how it works for social media; get people to look at your content, and they might just get more engaged with your product.
The same counts for the colourful application icons on your smartphone, and especially the notification marks for these apps that attract your attention and engage you with their their product.
Sources for further reading
- Morton J. Why color matters. COLORCOM. Available from: http://www.colorcom.com/research/why-color-matters. 2010.
- Labrecque LI, Milne GR. Exciting red and competent blue: the importance of color in marketing. Journal of the Academy of Marketing Science. 2012 Sep 1;40(5):711-27.
- Asadollahi A, Givee M. The role of graphic design in packaging and sales of product in Iran. Contemporary Marketing Review. 2011;1(5):30-4.