Have been musing this case again and to my mind Sam's colours are crucial and often not included in drawings such as the Bufora article. I am now even more inclined to believe 'Sam' was inspired in some way by Bubbles the Test Card clown:
"The most iconic image, introduced in 1967 with the advent of colour TV, was called Test Card F. Its designer was a BBC engineer called George Hersee and, for a dummy run, he had included a picture of his eight-year-old daughter, Carole, at the centre of it. The BBC decided that replacing Carole's picture with an adult model was too risky – they needed something timeless, and 1967 fashions weren't exactly built to last. So Carole went into a photographer's studio: the result was the familiar image of a girl with an alice band, playing noughts and crosses with a rather terrifying toy clown, surrounded by mysterious test graphics. Hersee was, unsurprisingly, teased at school and, to her discomfort, the image was used on a daily basis until 1998. Now living in the New Forest with two daughters, she can claim to have had more screen time – around 70,000 hours – than anyone else in British TV history."
https://www.theguardian.com/tv-and-radio/shortcuts/2012/apr/22/the-test-card-girl-and-clown
The child witness therefore could easily have seen Bubbles on a colour tv, even if just in a tv shop. The colours and description of Sam closely match those of Bubbles bar the discrepancy in height. Also "all colours" has a parallel with the first all-colour tv sets, in fact you could say Bubbles was in "all colours".
So if we are to accept the young girl was truthful about what she believed she had witnessed then I feel it was either the recollection of a vivid dream inspired by Bubbles that her young memory confused with a waking memory or they witnessed something so 'alien' that her mind desperately 'clutched' at images from her subconscious to try to explain what she was seeing.