Conditions: storm cells and clouds moving into the valley moving w/sw. dark clouds to the nw not quite over our location. fluffy white clouds above us and to the west--further west darker clouds moving sw. my fiancee and i check the radar and we enjoy watching storms roll into the valley. we were walking in our parking area--walking west. we were comparing the darkness of the sky to the nw to the fluffy whiter clouds overhead and west of us--disappointed that we probably wouldn't get any of the "good stuff" (bad weather). directly in front of us--roughly 30 yards or so, a concrete wall and another set of 2-story condos. facing these apartments and continuing to walk west--we notice a black object come into view over the condos. my immediate reaction, as i said to my fiancee, was that we were witnessing a not-so-impressive flight of a black garbage bag. after a few more seconds of observation--we observed that it wasn't changing shape, it wasn't fluttering, and based on ground conditions--seemed to be moving semi-perpendicular to the wind direction. strange--but, you-know--a bag right? a few more seconds went by and the object hadn't jostled or changed shape or changed direction; in fact, it seemed to be moving in a very straight line and to nw. no variation in its movement could be detected from where we were watching. a few more seconds and we both concluded that it had a definite rigid shape quite unlike a garbage bag--almost a very thin triangle. in addition it hadn't changed speed and was moving at a very consistent speed--no variation and smooth as could be given the wind turbulence on the ground. i suppose i could guess it was moving 45-50 mph but i can't really say for sure. we continued to watch, and followed it----we faced west at first, faced north as it moved along, and finally we ended up facing nw as it continued to move. again--no changes in direction or speed, it was if it had somewhere to be. as mentioned before the clouds just north and nw of us were dark--and as it began to move in front of these darker clouds it remained jet-black. i want it to be clear that the object remained black as it moved in front of darker clouds. still watching--and at this point moving more away from us than closer---it's color changed from black to white. instantly. i figured that an object might look black in front of whiter clouds then white amongst darker clouds, but we concluded that it should have had a white appearance as soon as it was in front of the darker clouds; however, it remained black for a certain period of time against the dark clouds until--in an instant turned white. very white. it kept moving in the same direction but was becoming fainter (in size) but still very visible as a white speck among the storm clouds. at that point it looked as thought it had risen in altitude several hundred feet. i ran inside, grabbed my camera and by the time i had come back--it had vanished. a moment later we saw another whiteish object north and low in the horizon--which i got on camera but was not behaving like the object we had just seen and could have been actual trash. trash? i don't know. i've unfortunately seen plenty of trash in the phoenix skies, especially during monsoon season. i also understand that wind directions and speed changes at certain altitudes. but i also know that trash bag doesn't "fly". this thing flew in one direction--but no other motion could be detected and no quick variations in altitude. it seemed to be moving in one direction, slowly climbing, not bothered one bit by the gusts and changes of wind direction. peculiar to say the least. the video attached is of a plane (so my fiancee and i think). there is a small airport just 1 mile north of us and they do circle around to make their landings. by the time i took this recording we had lost sight of the object we did see which was much higher in the sky, further west (left in the video). again--the video is of something seen shortly after but much less strange and most likely a plane; however, it did look very similar to the object that we did see after it had turned white.