Out of over 70 piggies, I have only had one piggy who had an issue with me switching from light coloured neutral fleeces to a dark red one. None of the others ever had an issue with different coloured fleeces, mats or cosies/fleece caves in all colours or lively patterns.
But Cariad (Welsh for 'Darling') was a very timid soul anyway and decidedly not the brightest anyway. She sat there for a week watching all her group mates walking on the new fleece and blocking the exit so the others had to occasionally actually jump over her to get in and out of the cage. She would put her paw out but just didn't dare touch the fleece and I did have to contruct a little 'bridge' with an acceptable doormat... But in the end, even she got it and moved around on it the same as all her other mates.
If you have concerns, don't wipe down any furnishings so the cage is still bearing their scent, which will help with acceptance.