Here I am collecting together the various things you may find useful.
(I am using UK terms but if you use US terms think of tr as dc and it should be straight forward.)
I made the blanket for a three foot single bed and chose to work seven ripples as it would then just more than cover the top (3ft 7ins). The border added another foot to the total width.
When I chose to make the Spectrum blanket I could only find fifteen colours in the Stylecraft Special DK yarn to fit with the spectrum as here.
[Post containing details of those colours HERE.]
And I knew to make the blanket long enough I needed more so I chose to add Bluebell for ‘sky’ and Silver for ‘cloud’ imagining a rainbow.
I chose an interleaving 2-4-2 row pattern for blending the colours and since I had to start somewhere chose to start with four rows of Bluebell.
You then get 2 Silver, 2 Blubell, 4 Silver, 2 Emperor, 2 Silver, 4 Emperor, 2 Royal, 2 Emperor, 4 Royal ……………………………
After all the spectrum colours, I finished with Silver again, then Bluebell, ending on four rows of Bluebell as at the start. As you can see in the photograph above.
I wanted to square off the ripples so I could make a square border. The details of how I chose to do this is HERE.
I have repeated the part of that page which refers to creating a sound foundation for the border as specific to the Spectrum blanket
Here are some (I hope) helpful pictures.
I have shown only some of the stitches as I think this should be clearer.
You will notice that I chose at this point to use Bluebell for the ends and beginning of the sides. This was because using the same colour for this part would hide any iregularities. I chose Silver for the rest of the sides as I thought of this as the colour that bordered the spectrum part.
It is worth mentioning here that I experimented a bit with using three and four trebles into the gaps at the sides and decided that three was not enough but four was slightly too many. Four was closer and to my mind the logical amount and so as I wanted a firm rather than fluid border, I went down a hook size from 5mm to 4.5mm.
At the top
The thick arrows represent the granny 3trs. I have shown the ones worked between those of the top row of the fill-in in white. The yellow ones are the ones worked into gaps in the final row of the main blanket. This general pattern will have been repeated all along the top.
5trs in the end stitch are to turn round the corner then I work 4trs into the obvious gaps down the side as shown.
For the Spectrum blanket I worked all the ones shown in the Bluebell and then continued down the side with the 4trs in the Silver yarn till I got to the last Silver stripe when I went back to Bluebell.
At the bottom
This shows how there are five lots of 4trs in Bluebell for the Spectrum blanket after you have finished the Silver at the last double Silver row.
Again there are 5trs in the corner stitch and two granny 3tr groups worked between those of the top row of the corner fill-in. There are three worked into the original bottom edge of the blanket but two of these are worked into gaps and one into the chain that the middle 3trs had been worked in on the first row. (As you should be able to see in the picture). With five granny 3tr groups worked between those of the top row of the main fill-in. Then back to the three shown in yellow and so on.
This does give you the same number of stitches along both the top and bottom edges.
You now have neat continuous stitches to form the basis of a rectangular border.
I started with a row of trebles in Silver all the way round for the reason given above then starting with Emperor, I made a row of trebles in each of the spectrum colours in turn. To resemble a rainbow where red is on the outside curve.
I used 5trs into the central stitch of the 5trs of the previous row at each corner. This does not give a perfect square corner but is about right.