Why does an yellow object absorb blue instead of all colors other than yellow? I've always thought that an object appears to be of certain color X because it absorbs all other colors and reflects only X. But my current textbook(and some quick googling) tells me this is not the case. 
As far as our eye is concerned, there is no difference between "yellow" and "red + green". This is because the cones in our eyes respond the same way to both of them. I guess I understand this.
But I still don't get why an yellow object would absorb only blue and reflect "red + green". Makes me wonder what's happening to all other frequencies... Appreciate any help. Thanks!
EDIT : This question is not a duplicate as I'm not asking about simple biology / physics of color. I think my question is specific : differences among "yellow", "red + green", and simple yellow reflection of an yellow object.
 A: It's worth emphasizing something that you say: "As far as our eye is concerned, there is no difference between 'yellow' and 'red + green'".  The point is that our eyes perceive these distinct physical phenomena to be the same.  So there are multiple ways things might work out.
Yellow is a color of its own, independent of red, and independent of green.  Wikipedia tells me that yellow is a color of light with wavelengths between roughly 590 and 560 nanometers.  However, the excellent link @anna v points to explains that the human eye might perceive different combinations of light (with different wavelengths) as being the same color.  In this case, it is possible to stimulate the human visual cortex to perceive yellow with a single wavelength of — say — 575 nanometers.  But it is also possible to stimulate the human visual cortex to perceive something we experience as yellow with two different wavelengths of — say — 675 nm and 540 nm at the same time.
So an object illuminated with white light may absorb every wavelength but 575 nm (including red, green, and blue), or it may absorb every wavelength but 675 nm and 540 nm (including yellow and blue).  Or maybe it'll absorb only wavelengths below 520 nm (so just blue).  Our eyes can't tell the difference.
A: @Mike already explained the color bit, but no one yet did quite answer "what's happening to all other frequencies".
Typically, for pigments, those photons get absorbed by the material of the object and converted to heat, as detailed in a very accessible manner in The Physics Classroom and here.
Another way photons can "disappear" is through destructive interference and that's indeed another possibility to generate color, the so-called structural color, generated by microscopic patterns in the material.
And the photons might also be not "destroyed", but rather be sent somewhere else (instead of your eye), as happens in the Rayleigh scattering, which is responsible by the most characteristic hues of the sky.
