Starting out, all open vehicles are sprayed inside and out in the standard color by the factory. The section of the driver and co-driver (in front of the body-joint) is sprayed white. Gradually this disappears. probably because the loss of air superiority. The white can be seen from above.
Grey is the standard color when production starts. Underneath is a rust red primer coat.
Before the war there are some camouflage attempts with brown. It is not seen often.
February 1943 the OKH issues the order for dark sand yellow. Additional color dark green and dark red brown is supplied in paste form (one report of powder). Pattern and application is free. Due to an explosion of variations August 19th of 1944 sees a new order of the OKH: all vehicles are to leave the factory in the famous ambush scheme (Brown and green stripes with many little spots in contrasting color). October 31st sees a new order: primer only, the color is put on (mainly by brush) in the field. November 30th the last known order says: dark green base color. The camouflage is ordered to be mostly Red-brown and secondary sand.
Black and white pictures are not the best source for judging, is it green or grey.
In the shortage of everything, color variations are logical. There are even stories on captured Italian and other paint. Field over painting and even sprays are common. Different war-theatres, different seasons, different foliage demand different colors and alterations in camouflage scheme.
Paint-patterns are often adapted to the kind of unit that the vehicle is used by. Vehicles directly working with tanks use their patterns.
The additional paint is supplied on a paste basis. It is to be mixed with gasoline or diesel fuel. As that becomes scarce in a lot of situations water is often used as an alternative. This coat of paint is of course weaker.
Paint changes color by the working of (ultra-violet-)light.
Basic colors can vary enormous through all these factors. Wear and tear of dust, sand and bushes and of servicing and mounting the vehicle can bring the rust-red primer or even bare metal to the surface. Camouflage, usually with foliage, is essential in the later part of the war. In some cases mud or whitewash are used. When paint or white-wash itself is in short supply, the front only is painted. In all this over painting and spraying, unit and tactical markings have to be repainted. Often the areas of markings are simply skipped. Operational frontline units often over paint everything. Many pictures show only crosses. The first markings to disappear are number plates, weight-specifications-table (used for rail-shipping), tactical symbols and unit markings.
In April 1944 Guderian, then inspector of armored troops, orders all AFV's in armed formations to adopt the numbering seen on tank-turret sides. There are units that use the 3-digit number system on m.SPW's prior to the 1944 orders. 6. Pz. Div. did.
When the role of mobile artillery evolved, it becomes custom to hide or dig in a vehicle. That required a paint scheme that helps hiding the vehicle in forest sides and bushes.
The surviving 251 in Vienna and the alongside found two 250's are SS vehicles. On determining the remains of paint and markings it is found that the vehicles were first issued to the Wehrmacht and later transferred to SS. This is done by over painting the markings and one of the three (tank-style) digits. A unit marking of the 5th SS Pz.Div. is done in light green.
The normal tank-unit vehicle numbering is seen in a 4 digit style after the April 1944 order that assigns random 2 digit codes to headquarter (Stabs-) units at Regiment Btl or Abt. for those units with more than one Btl. or Abteilung.
All this could mean simply the 1th or the 3th vehicle. On the introduction of a model of the puma (234/2) by Italeri in scale 1:35 the decals with the number 1111 were questioned. The 11th vehicle in a battalion? It could be right though. It could explain the 0 in these numbers. Germans are very punctual. When a battalion has more than 9 vehicles the 10th has a completely different number (e.g. 4 digit) Some Germans would fill out the numbering for they other 9!
A unit of the 506th heavy armor (Tiger II) even abandoned the battalion number. The unit's crumbled faster than they could supply the tanks.
Vehicles and guns where "numbered" alphabetically or got the squad letter and a number. Crews used first names, birds, cats and the spelling alphabet to name vehicle. Habicht (hawk), Falke (falcon), Löwe (Lion), Karl, Gerti, Paula, that kind of thing. It's most commonly seen in early war.