Iron is a good way to include them being the 'Red Mountains' without using the brighter Dornish terrainset palettes. The test is one of Tham's while the CTM blending was just a part of the thampack iirc.
Anyway, I think I've blathered on about this a lot but dividing the Stormlands into three vague regions for terrainset makes sense. Red Mountains (Iron, RM ts, Alt Storm ts, Pebbles, whatever it may be), Cape Wrath (Brown Storm ts) and Northern Stormlands (II ts, East Islands ts, Reach ts, MotM ts). The Red Mountains have the difficulty of sitting between three TS colours. Finding a subdued/dimmer palette - Stormlands is dour and gloomy when compared to Dorne - rather than trying to blend stripes into RM ts and Dorne ts would be a better solution to the issue at hand.
Now that's not to say I don't like the striations, I just think they could be more subtle, like how the pebbles aren't immediately identifiable but create interest or the grey light stone patterning that used to be above wullvillage3.
Honestly, I'm of the opinion that Stormlands terrainset should be used as a detailing block everywhere outside of old forests (tannin staining and less obvious rock formations, easier to hide caves and CotF hollow hills) and Cape Wrath and maybe in the north. Dunino Den and other areas of Scotland feature a red brown stone which looks similar to Stormlands ts. Imo, it fits well with these two settings and otherwise shouldn't cover an entire region that for the most part doesn't have its dark appearance, instead red and pale stone is described.