If you see the % marker as red is because the Crew is not trained for the specific Tank.
The effects of having a crew non specialized on the Tank they are handling affects in multiple ways:
If the whole crew was driving a tank different from the one you transferred them on, the general performance of the tank will be poor; lets assume you are transferring your crew from a medium tank to a heavy tank, the driving skills of the driver will be lower and the capacity to detect enemy tanks by the commander will be lower as well, same applies to all the other roles of the crew members. (Imagine transferring the crew members of a private Jet into an Airbus 380!)
There are other scenarios that can happen, for example, mixing crew members non-trained for the tank and others who are, in this case it will depend on the role; so for instance if the commander is trained for the tank you are using, but for instance the loader is not, you will spot enemies normally, but your re-loading performance will be poor. Take into account that the Tank Commander influences the whole crew, so a more specialized Commander will result on a better performing tank.
You can always re-train your crew from tank to tank, by right clicking your crew member, and selecting the Training tab of their profile, there you select to which tank you wish him to be re-trained; remember there are different types of training (50%-free, 75%-20000 igc,100% 200Gold) and as you might have noticed they also greatly influence the performance of the tank. Note however you can not transfer crew from Tank Destroyers into Light-Medium-Heavy Tanks, nor from SPG's to any type of tanks and vice-versa.
It is generally recommended to have your crew at least trained to 75%, if this is not possible try to train your commander and driver to 75%, they influence the general performance of the tank more than any other member.
If you want further info on crew members, their roles and effects on your tank read: Crew- World of Tanks
The formula for calculating the effective armor rating in world of tanks is:
Armor tickness/cos(impact angle)
With an Angle of > 70% always leading to a ricochet, this leads to this table:
Impact Angle Effective Armour Thickness
0° 100%
10° 101.54%
20° 106.42%
30° 115.47%
40° 130.54%
50° 155.57%
60° 200%
70° 292.38%
> 70° Ricochet
(source: world of tanks effective armor rating)
Which angle to chose depends a lot on the tank you're driving and how it is armored. But in general, having at least a slight angel towards your opponent is a good idea.
For example, if you're driving a tank with 100mm front armor and 80mm side armor, a 45% angle on both front armor would give you an effective front armor of 141mm and effective side armor of 113mm. In this example having the opponent hit your side at an angle would still give a lower chance of penetration than hitting the front armor at a straight angle.
However if the situation is the same, but the tank you're driving only has 60mm side armor this means the effective rating of the side armor would be 85mm, which means you're giving your opponent an opportunity at an easier penetration, in this case it would be still good to present yourself at an angle, but less than 45%. (a 30% angle in this case would give you 115mm front armor and 120mm side armor)
Best Answer
Yes, you can totally hit tanks without them being visible on the map. This is sometimes done by artillery or even tank destroyers. They scan the area for the usual hideouts, look for movement or gun trace, and fire.
Tanks that are hit by splash damage or direct hits will not be lit up, but you will know when you get a kill.
This is why it's very important for arty and base defenders to change positions, if they have been spotted by an enemy scout. Good enemy artillery take note of your position. They don't need to actually see you on the map, as long as they know your last position.