Cast off help!

Hi

I am knitting squares for a blanket. My latest square is a vertical zig zag pattern in 2 colors A + B
Last 2 rows were:

K1A (K3B, K3A) x5 K1B
(P3A, P3B) x5 P2A

Next instructions are to cast off knitwise into back off stitch, I have cast off like this before but never with 2 colours.

Any help appreciated.

Thanks

Sounds like a pretty pattern.
You can cast off using each of the colors in the same way that you’d use a single color. There’ll be a little continuation of one color into the next bind off where the yarns A and B meet but I don’t know a way to avoid this with the usual cast off. Carry the unused yarn behind each stitch so that you don’t have floats at the edge of the square.
You could cast off in one color and avoid the problem. Did you cast on with one color or with both A and B?

I did the cast on in one color A

OK, good. Then you could either cast off in the same color or cast off in the opposite color. Either will work of course depending on the look you want to the square. Since the side edges are in contrasting colors, you might use the opposite color from the cast on for the bind off.
You may also want to think about the color and method that you’re going to use to join the squares.