Unfortunately the Hu126 isn't the closest much for the RLM76 I'm afraid. The colour was of blue tinge, also of blue-green one. The Hu 126 is just a grey colour.
The mix ratio is a proportion of paints you have to use in order to get a final colour. In other words, it is a clue how much of each colour has to be used. For instance the mix ratio is 1:1. It means that you have to take one drop of one paint and one drop of the another one. in general - 50%/50% . I said "one drop" but of course it isn't enough for painting of all surfaces of a model. It can be used for a pilot's stick of 1/72 scale only.... . But I'm sure you grasped. So for painting of larger areas you need to use more of the mixture. Let's see.... for example... 1cm³ of one paint and 1cm³ of the another one.
If the ratio is 1:2 for instance ... it means that you have to use one drop of one colour and two drops of the another, viz, there it has to be the double amount of the second paint. Have you caught?
As far as the interior colour id concerned.... the Hu32 is said to be an equivalent for RLM74 Graugrun ( FS36118 ) rather. But it is too dark. The interior paint should be RLM66 ( FS36081 ) Schwarzgrau - Humbrol67 or Humbrol79. Here you are comparing of both FS numbers and Humbrol colours. You can notice that the Hu32 and Hu163 are of green tinge while the Hu67 and Hu79 are of grey tonality.