Standard bath panels measure height and length
It should be noted that if only a side (long) panel is required it is unlikely that it will marry with an existing end panel
End panels should be measured if the bath is over the 700mm standard size.
Corner bath panels are not interchangeable, unless the make and model are known or a digital photo supplied it is unlikely we will find a match
Colours: occasionally the colour is stamped on the back of the panel. The best alternative is to investigate the toilet or basin for a make, this and an indication of age should help narrow the possibilities for us to send some colour samples for confirmation.