The chosen font has strokes thinner than 1px at the chosen size. Every rendering engine has its own way of handling this kind of broken design, your engine uses subpixels to show accurate stroke width, therefore the weird colors.

[Edit:] Those ultra-light fonts are a very unfortunate fad that probably began with the introduction of iOS 7, maybe even earlier with MS's Metro, contrived and perpetuated by people who don't read but who look at texts.