If you have inserted yellow to a pixel layer which already had a bitmap image and the yellow partially covers the older other colors you are out of luck.

If the inserted yellow is the only color in the pixel layer and you do not want to repaint that layer you can insert effect Color overlay which changes the color of the non-transparent area. Effect can have layer mask which gives a possibility to keep intact those areas which have not got yellow.

You can also recycle (copy & paste) the content of the yellow layer via Affinity Photo which has well working Hue & Saturation adjustment. There you can make a selection if the yellow isn't mixed with other colors in the same layer, but is a separate island.

If you have inserted yellow to a pixel layer which already had a bitmap image and the yellow partially mixed with other already existing colors you are out of luck. The only ways out are Undo, revert to a backup or redraw the destroyed layer from a start.

If the inserted yellow is the only color in the pixel layer and you do not want to repaint that layer you can insert effect Color overlay which changes the color of the non-transparent area. Effect can have layer mask which gives a possibility to keep intact those areas which have not got yellow.

You can also recycle (copy & paste) the content of the yellow layer via Affinity Photo which has well working Hue & Saturation adjustment. There you can make a selection if the yellow isn't mixed with other colors in the same layer, but is a separate island.