2 corrected a typo, improved formatting
source | link

What you need to do is to get R2' and R3', the projecttionsprojections of R2 and R3 respectively, with only the attribute sid. Then take their intersection for R4:

    R1  := ...
    R2  := ...
    R3  := ...
    ---
    R2' = πsid(R2)R2' := πsid(R2)
    R3' := πsid(R3)
    R4  := R2' ∩ R3'
    R3' = πsid(R3)
   R5 R4 := R2' ∩ R3'
    --- 
    R5 = ...
    R6  := ...

What you need to do is to get R2' and R3', the projecttions of R2 and R3 respectively, with only the attribute sid. Then take their intersection for R4:

    R1 = ...
    R2 = ...
    R3 = ...
    ---
    R2' = πsid(R2)
    R3' = πsid(R3)
    R4 := R2' ∩ R3'
    --- 
    R5 = ...
    R6 = ...

What you need to do is to get R2' and R3', the projections of R2 and R3 respectively, with only the attribute sid. Then take their intersection for R4:

    R1  := ...
    R2  := ...
    R3  := ...
    R2' := πsid(R2)
    R3' := πsid(R3)
    R4  := R2' ∩ R3'
    R5  := ...
    R6  := ...
1
source | link

This is indeed the error:

    R4 := R2 ∩ R3 

If we try to get the intermediate results of your solution, we get:

R1:

bid  bname      color       sid  date
---  ---------  -----       ---  ----------
102  interlake  red         31   8/11/2014
103  clipper    green       22   7/05/2014
103  clipper    green       58   8/11/2014
104  clipper    red         22   8/10/2014

and then:

R2: (only the 'red')

bid  bname      color       sid  date
---  ---------  -----       ---  ----------
102  interlake  red         31   8/11/2014
104  clipper    red         22   8/10/2014

R3: (only the 'green')

bid  bname      color       sid  date
---  ---------  -----       ---  ----------
103  clipper    green       22   7/05/2014
103  clipper    green       58   8/11/2014

R4 will be an empty relation because nothing can be both red and green. Of course that is not what the exercise asks for.

R4:

bid  bname      color       sid  date
---  ---------  -----       ---  ----------

What you need to do is to get R2' and R3', the projecttions of R2 and R3 respectively, with only the attribute sid. Then take their intersection for R4:

    R1 = ...
    R2 = ...
    R3 = ...
    ---
    R2' = πsid(R2)
    R3' = πsid(R3)
    R4 := R2' ∩ R3'
    --- 
    R5 = ...
    R6 = ...