Alexandre, here we have one of the major dilemma's in modelling. Some people are very interested in historical accuracy. They will do what ever they can to get a model as accurate as possible. And to do this, they often rely on the advice of others who may know more about the particular subject. The problem is, those who offer that advice are often ridiculed as rivet counters.
At the other end of the scale, some simply like to build and are not bothered at how accurate it is. On occasion, this people will be ridiculed by those that believe that every kit should be 100% historically accurate.
The problem is, for those who are happy to offer any advice in a helpful manner don't know what reception they will receive.
My own view is this. I strive to be as accurate as possible, and I will welcome any comments, advice or suggestion's. However, if you want to build a 251, paint it pink and put it in the Somme in 1916, then its your plastic, do what you like.
That being said, Roy is 100% correct. The 251/21, and its the /21 that's the important bit, did not enter service until the latter half of 1944.
The inside and out would indeed have been painted the same colour, but of course weathering would have effected the outside more than the inside. And I recall reading one account where a museum stripped a 251 and found something like 9 different shades of DY.
How to want to use al of this is entirely upto you. I hope you will take it in the fashion it is given, with the intent of being helpful and not patronising or arrogant.
aside from that, nice looking build. I am a massive 251 fan and never get enough of them. Which kit is this one.