This is how it goes with me: I find an old Rowan pattern that I like a lot - a cardigan in Magpie aran with a sort of fair isle border around hem and sleeves. I have two colours of Magpie and stalk some more on eBay. The Magpie is too expensive so I buy Jaeger Shetland aran instead. So far so good. I then swatch the fair isle pattern and realise it's going to drive me mad. At one point there are four colours in one row and it's knitted flat, so I'd have to be purling all those colours as well. That's out.
Then I think: I could make a yoked jersey following Elizabeth Zimmermann's EPS. So that's what I'm now doing. I think I'll add a small border around the hem and sleeves just to use up more Magpie and because I may not have enough of the main colour in the Jaeger Shetland.