Yeah, well its fine, the HUD already have features that only was present in the JA37Di variant.
It would most likely be painted in fully green. As can be seen here (the inner pylons):
http://www.airvectors.net/avvig.htmlThe RB24s are built into the main plane model (ja37.ac).
What you should do is look into how the M70 are put on, thats the best way to do it, and some time in the future I will probably convert the RB24s to use that system instead.
Look at the file Models\Armament\Weapons\M70\rocketPod.xml for loading the missile.
Look at line 2492 in Models\JA37-Viggen.xml there you will see how it is included. Notice how the in the select animation JSBSim weight decides if they should be shown or not. Add the RB99 in the same manner, one model and one select animation for each pylon. Should automatically work for MP also.
Then you have to look at line 1034 in JA37-Viggen-set.xml and include the missile in the same way the other armament is included, make sure to use precisely the same weight as in the select animation you added.
Then in Nasal\ja37.nas at line 252 there is a if statement for the RB24, make a else statement after it that works in a similar way for RB99.
In the same file at line 308 there is an ifels statement for RB24, make one for RB99 and make sure you use the exact weight that you used earlier.
In still the same file at line 824 you should make sure that it gives another message when you fire the RB99, but from the look of your screenshot it looks like you already did that.
At line 1918 in Nasal/hud.nas there is an ifels for the different weapons, to tell the hud range indicator of the of their minimum and maximum range. Add the RB99 there.
Line 1706 in the same file makes the HUD show what kind of weapon is selected, add the RB99 there.
At line 707 in ja37-aerodynamics.xml is a table with the weights and the drag of what is mounted on the pylon, add a line for the weight and the the drag you estimate (not the same as the drag for the rb99 in the aim-9 code, as that is factored onto another factor (is why its 1 for rb24 and 2 for M70), so look at the numbers already there and make an estimate). There is 3 more tables like that underneath, one for each pylon.
I might have forgotten something, but that should get you started.