I’ve managed to solve the problem completely, at least for myself.
I needed to round corners of my widget so I made an alpha mask in Photoshop. Then I used Retainer Box with a transparent material. My colors were washing out (first screenshot).
Then after a following manipulation with the material, I managed to make it just the right color as it was without the Retainer Box (second screenshot).
And here’s what I’ve done in the material to make it happen (white texture sample is my alpha texture for corners).
Hope it helps!