Red color is perfect pixel density, the quads you see represent pixels. Aka one quad is one pixel, so imagine you have one rasterized picture in photoshop and scale it up until you see the individual pixels on the picture and it looks ugly right? So the lower pixel density (blue color) the bigger the pixels are which leads to less details in shadows and/or shadow artefacts. And other way around, the more pixel density you have (orange/red) the smaller the pixels are which produce more details and less shadow artefacts.
The difference in color you see is because your uvs arent completely 100% stretched out, but if its that small difference in color its not a problem. Your problem is the ligthmass settings.You have **Static Lighting Level Scale **on 1 which will not produce good results. Also you need to build light in production quality (check screenshot).
Try with those settings:
Set the build lighting quality on production (check the sceenshot)
Lightmass settings (check screenshot)
Static Lighting Level Scale: 0.1
Num Indirect Lighting Bounces: 5
Num Sky Lighting Bounces: 5
Indirect Lighting Quality: 1 (Max 4)
Indirect Lighting Smoothness: 0.6
And UV resolution on 512 (the color should be orangeish/redish).