I live in Portugal. I have a neriun oleander variegata whose leaves are on certain branches completely yellow. How can I get the whole bush to give leaves whid this color?
You can't - it's just a genetic thing. The genetic programming for each branch and leaf may have minor variations, so some will be highly variegated, others less so and some may be yellow or even plain green. That's assuming the plant is healthy and growing well - if it's not, the yellow leaves may be about to shrivel and drop off.