A wonderfully bright and cheerful floral composition Barbara, however for me the colour seem to be somewhat over saturated, below is a little snippet which I gleaned from some past notes which I hope you will find useful.
'Yellow & white flowers can be tricky to photograph (especially on bright sunny days). These colours tend to overexpose easily. For better results try to photograph them in the shade.'
The truth is that bright yellow reflects more light than an averaged tone 18% gray card, green grass or matt green leaves – although not so much as white. This means that using an uncorrected TTL in-camera reading will result in dingy looking yellows that are underexposed. Essentially, you need to open up on the metered reading. This can be done either by manually metering a matt green leaf lit in the same way as the yellow flower or by using exposure compensation (need to use a + value to increase the exposure to compensate for bright yellow). Depending on how much of the frame is filled with yellow and whether there are other tones in the shot, the exposure increase can be as much as a stop or more.