Seems to me there are two separate issues here. 1, A minority group of councillors have acted unilaterally to the exclusion of the rest of the council without prior discussion and agreement and that in itself has to be unacceptable for a number of reasons. 2, A way to chastise this unacceptable behavior is now being sort through H&S and COSHH regulations.
What a wonderful 'big stick' opportunity H&S and COSHH regulations can provide thesedays in bringing persons to heel, but it might be better to address such unaceptable behavior in more direct manner using the rules of transparency and openness in council debating and decision making.
If the paint used was a reputable and approved ground paint it will have its own COSHH sheet complying with the necessary regulations showing it to be safe for the purpose it is intended. If the paint wasn't approved ground paint then some legs do need slapping especially if this is in a pubic area.
A risk assessment is expected to be proportional in relation to cost and any reasonable potential of risk, the key word being reasonable. It is hard to imagine how any risk at all might be involved in the action you describe, unless of course there are indeed bulls in the field or it is maybe in use for some other potentially hazardous activity you have not mentioned but then if it's a public area it sounds more like it might be a sports ground or similar and therefore as a public area must surely be by default already void of any such H&S risks or hazards.
Most people have common sense and one would hope that three councillors might be able to summon sufficient between them. Therefore if there is obviously no risk involved then the key words 'cost -V- reasonable potential' would come into play. A risk assessment carried out by unqualified laymen has no real merit and would be unlikely to be accepted by insurers or a court if it came to that. Therefore a qualified RA should always be commissioned whenever common sense indicates tha a risk may exist. However the objective of a risk assessment is to reduce any identified risk to what might be considered reasonable -V- costs involved.
Personally, considering that this is already a public open space I can't see that the action you describe would reasonably justify the cost of a professional risk assessment as it sounds like you might just end up with a high fee in exchange for a blank document.
Although I do fully endorse that the autonomous and possibly fractious behavior you outline to be very disagreeable.