CRM 2013 has gone gray scale icons and all the icons you use for your CRM 2013 custom entities should look the same.
Icons8 (http://www.icons8.com) has a large (1500+) metro-style icons perfect for this. They are free for personal and commercial, use, as long as you include an attribution and a link back to their site. Alternatively, you can purchase them at a very nominal fee of USD 149.
They are currently available at various sizes – 26×26, 32×32, 48×48, 64×64, 128×128, 256×256, and 512×512. Size 16×16 isn’t currently available, but re-sizing is permitted, even for the free version, and you can do so by using a batch image tool.
To change icon of entity
1. Go to Customization and select the entity record
2. Click on Update icons on the ribbon
3. Select the values for web application and entity forms –
Click OK and publish the changes.