1. The rhetorical technique used in "If I had known that I would never catch the eye of the times, I would have bought more rouge and painted peonies" is: irony.
The meaning of these two lines of poetry is: If I had known that light-ink landscape paintings would not be appreciated by contemporary people, it would be better to buy more rouge to paint delicate peonies. On the surface, the poem is meant to cater to the people of the time, but in fact it is an expression of anger and irony towards the people of the time who only valued glitz and wealth (the richly colored peonies) and did not recognize talents (the sublime artistic conception in front of them). It also reveals the poet's lament that his talent was not appreciated.
2. "Had I known that it would not catch the eye of the times, I would have bought more rouge to paint peonies" comes from "Inscriptions on Paintings" by Li Tang, a poet of the Song Dynasty
"Inscriptions on Paintings" Song Dynasty
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Li Tang
In the clouds, in the mist of the village, in the rain, it is easy to see but hard to do.
Had I known that it would not catch the eye of the times, I would have bought more peonies painted in Yanzhi.