Batch Numbers
Personally, I'm an enamel guy. I've never had a whole lot of luck with acrylics but the same rules pretty much apply with both types of paint
Paint is made in batches, hence every time you buy paint there's a batch number on the bottle.
One problem is that the color can differ from batch to batch. Usually it's quite subtle and not noticeable but at other times it means repainting the entire model.
I usually make a practice of buying several bottle of the same color/batch to ensure uniformity of color
Below is an example of batch inconsistency,
I had bought two bottles of f/s 36440 Light Gull Grey a couple years back and unfortunately, one had gone bad. I went to the spare time shop and grabbed another bottle hoping for the best. You can see the color variation between the wings and the fuselage. Subtle, but much more noticeable to the naked eye.

Color Drift
Another problem you may run into from time to time is color drift. indicative of this is the paint seems to thicken and require thinning more often
As you've probably seen, when paint sits on the shelf, the pigment and the carrier separate. What can also happen is the colorant in the pigment can separate and if not thoroughly remixed every time you use it, the color shifts or "drifts" a small amount.
Over the course on many uses it can be quite dramatic when compared to a new bottle.
Below is two bottles of Floquil Gun Metal. One I bought 40 years ago and have used in miniscule quantities. The other lighter color I bought when I found out Floquil's been discontinued.
The newer bottle I gave a quick shake by hand and as can be seen, there's still a large amount of pigments around the bottom of the bottle where the darker bottle is thorougly mixed. (metalics are more susceptable to this due to the relative weight of the pigments)
I went out and bought a small paint shaker and after shaking the bottle by hand for a few minutes (and a warm water bath if it's cold) I'll throw it on the shaker for a few minutes to ensure complete mixing. The photo on the right is thorougly shaken (not stirred).
Also noticeable is the change in color once fully mixed. The two bottles appear to be closer in color but still different.