As Bulb don’t generate any of their own power, they can’t “Greenwash” any of their energy, however this doesn’t necessarily make it as green as it could be.
REGOs are created for every unit of renewable energy created. By buying REGOs, you are effectively allocating that created unit of renewable energy to your supply. In that sense, all of the energy that Bulb supplies is green.
The issue with this lies where companies have some renewables (often because they have to), which they don’t “use” for their customers (as their customers don’t pay for “green energy”), so they sell off those REGOs to companies like Bulb. As those REGOs are basically always going to be there, there’s potentially no additional demand being created by buying them. If they didn’t get sold, the same amount of renewable energy would be being supplied.
Then there’s actual greenwashing where companies (like Shell) generate dirty power, then buy left over REGOs from other companies, effectively replacing their dirty grid energy with renewable energy for their customers. Their dirty energy is still going into the grid and being sold to customers who don’t know/don’t care about paying for renewables.
If you want the greenest renewables, you want to buy from a company that owns enough renewable generation for all of their customers, or has direct purchase agreements with renewable generators to cover all of their customers’ demands. Although this still involves REGOs, they’re primarily a formality in this case to show/ensure that the energy company is “getting” all of the renewables for their customers.