Experiment here. I'm haven't synesthesia but I do have some associations of colour and their brightness/darkness with days of the week (these don't match those in the article). If anyone wants to do the same, mine are below in base64 (emacs has base64-decode-region or you can do it online). I'd be interested if there's a correlation.
c2F0dXJkYXkgLSBkYXJraXNoIGdyZXksIGJsYWNraXNoCgpzdW5kYXkgLSB3aGl0ZQoKbW9uZGF5
IC0gZGVmaW5pdGVseSBzdHJvbmcgcmVkCgp0dWVzZGF5IC0gc29tZXRoaW5nIGRhcmssIGxpa2Ug
dmVyeSBkYXJrIHB1cnBsZQoKd2VkbmVzZGF5IC0gbGlnaHQsIHBlcmhhcHMgbGlnaHQgZ3JlZW4s
IGxpbWUKCnRodXJzZGF5IC0gZGVlcCBwdXJwbGUtaXNoLCBibGFja2lzaCwgbGlrZSB0aHVuZGVy
Y2xvdWRzICgndGh1cicgZ2l2ZXMgaXQgdGhlIGRhcmtuZXNzIGF0dHJpYnV0ZSkKCmZyaWRheSAt
IGxpZ2h0IGdyZXk=