Can't access time machine backups after rebuild

My Mac mini has been exceedingly slow for some time even though I had 400GB free of 1TB capacity so after backing up to time machine, I reformatted the hard drive and reinstalled Elcapitan. When I tried to access the backups after the reload they are red and will not allow me to access them.

I went into the permissions for the external drive and added Read/write permissions to the last 4 backups. However after adding those permissions, I still can't access those backups to restore the data. What can I do to correct this issue?

In addition, with only 55GB used, the Mini is extremely slow. I'm getting the pinwheel for almost anything. Any ideas?

Help!!! Suggestions?

Mini details:

Model Name: Mac mini

Model Identifier: Macmini6,2

Processor Name: Intel Core i7

Processor Speed: 2.3 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 6 MB

Memory: 16 GB

Boot ROM Version: MM61.0106.B0A

SMC Version (system): 2.8f1

Serial Number (system): C07L1111DWYM

Hardware UUID: 5FFB0249-3633-5387-9870-E283FFFC759D