My PS2 Slim plays music, black and blue discs, but doesn't play silver discs nor DVDs.

Pleas do not leave not helpful ratings when you have not even provided enough information to make a full diagnosis and try to be clear in your description of the action you have taken so far, we can only be helpful if we have the correct information from you.
By "I already opened it" do you mean you have disassembled or do you mean you have just opened the drive lid? if you have only opened the drive lid and your PS2 has had this fault since you bought it or is less than 12 months old you can make a claim against your Sony warranty, if however you have disassembled your PS2 your warranty is void. Pleas do not leave not helpful ratings when you have not even provided enough information to make a full diagnosis and try to be clear in your description of the action you have taken so far, we can only be helpful if we have the correct information from you. By "I already opened it" do you mean you have disassembled or do you mean you have just opened the drive lid? if you have only opened the drive lid and your PS2 has had this fault since you bought it or is less than 12 months old you can make a claim against your Sony warranty, if however you have disassembled your PS2 your warranty is void.

Ah I see!
Just looked at you're profile and realise you've already voided your warranty fixing a hardware issue Sony should have fixed for you.
Anyway the first thing to do is clean the laser, even if it looks clean you'd be surprised how often this can resolve disk read errors.
With this kind of drive you can clean it with a cleaning disk (a cheap item from any electronics/game/music store or supermarket for that matter), you can clean it with alcohol free CD wipes, you can also clean it with an air duster, often a helpful solution as it blows dust out of the entire drive. ***warning*** Do not clean the laser lens with household cleaning agents or isopropyl alcohol, the lens is made of polycarbonate and will discolour if subjected to isopropyl alcohol based cleaning agents and suffer micro-scratches if subjected to mild abrasives.

If cleaning the laser doesn't help or you've already used the wrong kind of cleaner you are going to need to either re-calibrate the laser or replace the CD drive. This will depend on 1: if you have damaged the laser with the wrong cleaning product (a common issue) and 2: what drive was installed at the factory. Some units where built with a drive that does not have an adjustable laser but most where, ocated on the topright side of the laser unit are two small silver screws, these are the calibration adjusters, make a diagram of there original positions and then start adjusting them half a millimetre at a time and testing your disks until they work flawlessly.
I wouldn't normally advise doing this as you need to disassemble your PS2 to get to the calibration adjusters but hay you've already done that before.
So, if re-calibration doesn't help you're going to need to replace the CD drive, there are several specialist retailers out there who sell replacement drives, removing the original drive will familiarise you with the procedure and also allow you to see the drives model number, if you enter the model number in to your favourite search engine you should find a retailer with that model unit in stock.

***As usual I am in no way responsible if you kill you're PS2 when you do this, it's not normally advisable to take it apart.*** Ah I see! Just looked at you're profile and realise you've already voided your warranty fixing a hardware issue Sony should have fixed for you. Anyway the first thing to do is clean the laser, even if it looks clean you'd be surprised how often this can resolve disk read errors. With this kind of drive you can clean it with a cleaning disk (a cheap item from any electronics/game/music store or supermarket for that matter), you can clean it with alcohol free CD wipes, you can also clean it with an air duster, often a helpful solution as it blows dust out of the entire drive. ***warning*** Do not clean the laser lens with household cleaning agents or isopropyl alcohol, the lens is made of polycarbonate and will discolour if subjected to isopropyl alcohol based cleaning agents and suffer micro-scratches if subjected to mild abrasives. If cleaning the laser doesn't help or you've already used the wrong kind of cleaner you are going to need to either re-calibrate the laser or replace the CD drive. This will depend on 1: if you have damaged the laser with the wrong cleaning product (a common issue) and 2: what drive was installed at the factory. Some units where built with a drive that does not have an adjustable laser but most where, ocated on the topright side of the laser unit are two small silver screws, these are the calibration adjusters, make a diagram of there original positions and then start adjusting them half a millimetre at a time and testing your disks until they work flawlessly. I wouldn't normally advise doing this as you need to disassemble your PS2 to get to the calibration adjusters but hay you've already done that before. So, if re-calibration doesn't help you're going to need to replace the CD drive, there are several specialist retailers out there who sell replacement drives, removing the original drive will familiarise you with the procedure and also allow you to see the drives model number, if you enter the model number in to your favourite search engine you should find a retailer with that model unit in stock. ***As usual I am in no way responsible if you kill you're PS2 when you do this, it's not normally advisable to take it apart.***