So sorry to hear that you are having to deal with this throat dryness and the recurrent colds. With regards to why it gets better as the day goes on, this could be for several reasons. The most likely is that post nasal drip (perhaps from an impending upper respiratory tract infection) that is falling down onto the back of your throat all night long as you are asleep. This drip causes inflammation and redness that makes your throat feel scratchy and just not right. As the day goes on, the mucous coming from your nose is able to be cleared more rapidly, and the normal saliva and secretions from your upper aerodigestive tract is sufficient to cleanse the back of your throat, causing it to feel better and be more normal.
Another thing that happens while you sleep is that, especially when you are starting to get stuffy in your nose from a coming cold, you will tend to sleep with your mouth open. This will obviously dry out the sensitive tissues at the back of your throat. This improves as you go through the day with your mouth closed, and swallowing appropriately. Finally, some people will also have issues with GERD, or acid reflux that can cause similar symptoms. Please see an otolaryngologist
(aka Ear-Nose and Throat surgeon
) or your primary care doctor
if the symptoms persist or become worse.