Picking up 2D collectables in an order?

Do the blocks have some sort of "BlockColor" enum-style property? If so, you could set up an array in the inspector to control the block order itself, then monitor the required next block that way?

You could consider adding something like this to the "player" doing the collecting:

Edit: be sure to build out the color array in your inspector.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public enum BlockType
 {
     Red = 0,
     Blue = 1,
     Yellow = 2,
     Green = 3,
     Black = 4,
     White = 5,
 }
 
 public class BlockGrabber : $$anonymous$$onoBehaviour {
 
     // Substitute your own enum for colors, this
     // will just show the idea.
     public BlockType[] colorOrder;
 
     // Then keep track of your progress
     private int nextColorIndex = 0;
 
     // Check if a given block type would satisfy
     // the order
     public bool $$anonymous$$atchesNextBlock( BlockType b)
     {
         // The index is only being incremented here,
         // so we shouldn't be getting any weird behavior
         //
         return b == this.colorOrder[this.nextColorIndex];
     }
 
     // Increment the index and see if that was  the last
     // color required
     public bool IncrementandCheck()
     {
         // Add to our index
         this.nextColorIndex++;
 
         // Then return whether this exceeds our color array
         return this.nextColorIndex >= this.colorOrder.Length;
     }
 }

Then adding a property with that enum onto your blocks themselves, adjusting the collision function appropriately:

 public BlockType blockType;
 
 void OnTriggerEnter2D(Collider2D other)
 {
     if (other.tag == "Player")
     {
         // We'll check the array on this object
         BlockGrabber grabber = other.GetComponent<BlockGrabber>();
 
         // See if this block's color is what the player
         // wanted to grab next
         bool match = grabber.$$anonymous$$atchesNextBlock( this.blockType);
 
         // If this isn't the right block, break out here
         if (!match)
             return;
 
         // If we're still here, then this block fits the bill, so
         // check if we're actually done
         bool done = grabber.IncrementandCheck();
 
 //            ++BlocksCollected;
 //            Debug.Log(BlocksCollected);
 
         gameObject.GetComponent<AudioSource>().Play();
 
 //            Collect();
 
         if (done)
         {
             Scene$$anonymous$$anager.LoadScene("Level2");
         }
     }
 }

The example array there will need to be populated within the inspector, but, if you've already done that, then the player might need to have their collection index reset on a new scene.

By default, inspector arrays are of length 0, so any index will throw an exception.