If you take specific shots of flowers which aren't a small part of a much bigger scene then an orange or red filter will bring out the difference between red and green much better than a yellow.

On the other hand if you live in a very sunny high contrast/bright and deep blue skies sort of area and take general scenes especially wide landscapes then a mid yellow filter might capture the gentle summer's day look better.

On most occasions in the U.K. a light or even mid yellow filter can render skies which look a little weak in my opinion but the word "opinion" is key because that is what it is - not an exact science with a right and wrong. What looks like a natural rendering of a sky to me may not be natural to you or you may simply prefer darker, more dramatic skies.

Finally I'd consider a yellow green and a green. The former for lightening scenes where green predominates and the latter where you might want to render green foliage much lighter and red petals darker. So the opposite effect to the red.

You'll never know what is possible unless you are armed with yellow, orange, red, yellow green and green.

Try the same scene with all 5 and examine the prints - probably the best way to learn how to gauge which filter is most appropriate for scenes in the future.

Have fun

pentaxuser