Continue with your original plan.
Here's what will happen:
1. You will have a complete gown and a new set of sleeves.
2. You will be able to wear them together.
3. You will be able to mix them with other pieces in your wardrobe.
4. You will be finished on time.
Those are all good things, yes?! Then, when you have more time, you can start hunting for a lighter shade of orange linen (or wool), or play with dyeing.
Orange and blue are opposites on the color wheel, and going one shade off of pure orange and pure blue will actually help "calm down" the color scheme. I
know you want it to look exactly like the picture, but given your time constraints and the fact that you already have enough fabric to proceed, why not go with the original plan? If you decide you don't like the blue sleeves with the darker orange, *then* go ahead and make the larger matching sleeves (do you have enough fabric? Those sleeves are *huge!*). The two gowns are very similar--except for the guards and the color and the sleeves, they could be interchangeable.
I think you're fine. Make the gown, then decide about the sleeves.
But maybe have some wine first.
