That would be a correct. Tamiya does not have a full color range instead they give you formulas to obtain the correct shade needed. The color you are looking for is RLM 02. If you have ascess to model masters paints they make a full line of all of the german RLM colors. No mixing and matching just paint away.
That’s strange, since Tamiya have a version of RLM02 in their paint range (XF22). That said, adding equal parts of XF49 would give you something rather closer to the generally accepted shade for RLM02, but maybe a bit too yellow. What part of the model is supposed to be painted in this colour?
The number following the colon [:] represents the proportion of paint. Thus XF-1:2 + XF-5:3 means 2 parts xf-1 [or %40] mixed with 3 parts XF-5 or [%60]. To make certain the proportions are correct, they must add up to %100. To determine if this is so,
add the two proportion numbers together - in this case, add 2 + 3 = 5.
divide 100 by the result of step 1. in this case 100 / 5 = 20.
The result of step 2 is the percent contribution of each “part” or unit of paint. In this case, each unit of paint is %20 of the completed batch.
20% [the contribution of one part] x 5 [the number of parts] = 100%. If it does not, you have made an error somewhere. CHeck your work.
To achieve these proportions, you can measure by weight or by volume. Use weight if you have an electronic scale with low capacity. There are a number of them at low cost which appear to be manufactured for measuring cocaine that have measurement in grams. The alternative is to use a pipette and count the drops of paint. This will not be exact, but will get mighty close to the recommended color.
My advice is that if you start mixing colors, get a bottle to keep them in, mark it with the formula used, make too much paint and keep it. It is almost impossible to start with the formula and mix a batch of paint that will exactly match a shade mixed at some other time. If you want to lighten or darken shade for artistic/weathing reasons, do so in the airbrush, not in the bottle.
Pharmacies have inexpensive, small, graduated plastic cups used for dosing medicine for children that are very useful for both mixing and working with paint mixtures.
That colour would be RLM02 Grey, the standard colour for German aircraft interiors (apart from fighter and some bomber cockpits from early-mid 1941 onwards):
BTW, an effective way of mixing small quantities of paint is to drip the paint off the blade of a small screwdriver into the airbrush colour cup or mixing vessel. If you mix both paints equally thoroughly, each drop will be of equal volume.