Hi
I’m a complete beginner at knitting and following a pattern which has so far been different colours on different rows, but the next row is this:
K4 Brown, K43 Red, K4 Brown
How do I knit with 2 colours in the same row? At the moment I’ve only got the red wool attached as it’s the last colour I used.
Thanks, and sorry if this seems like a silly question - very new to knitting! 
You can either carry the other yarn across the back - just catch it with the other yarn every few stitches, or use another ball of the brown at the other end, just twist the yarns round each other once so you don’t get a hole (there are videos which illustrate these things in case you need to see!). Using another brown ball makes more sense if you don’t need a thicker fabric or want to save yarn.
Thank you
As this is continued for a few rows (e.g. next row is the same but purl stitch) how do I make sure that the caught yarn that’s carried across the back is always on the ‘wrong’ side?
As long as you do not take it between the knitting needles it will stay on the wrong side. After twisting with the working yarn you might have to give the brown yarn a slight tug if you notice it is visible from the front side.