Faces in Sketchup have front sides and back sides. With the style you are using the default front face color is white while the back face color is blue. Best practice is to ensure only white front faces are exposed. There are a number of reasons for that. One reason is mixed face orientation makes texture/material management difficult. Another common reason has to do with rendering. Some rendering applications won’t render exposed back faces even if they have materials applied to them.
To correct the reversed faces you caan select them, then right click on them and choose Reverse Faces.
It’s best to stay on top of the face orientation as you are modeling. Don’t get ahead of yourself and start applying textures before the geometry is all correct.
Many users edit their template’s default style to change the back face color to something bright and annoying like magenta. That encourages them to fix the reversed faces if they occur. I use a green that isn’t especially annoying but stands out better than the default blue.