The group also warned that, without additional money, "noticeable changes" will have to be made to further education.
According to Colleges Scotland, the sector is facing a funding loss of £51.9 million - with this including £28 million of additional funding it had received in the aftermath of Covid.
This cash was used to provide financial support for foundation apprenticeships, mental health projects and wellbeing initiatives, as well as vital digital equipment for students to help with remote learning.
It comes at the same time as colleges are facing a series cost pressures, including rising energy bills, the cost of providing staff with pay awards and changes to National Insurance contributions.
Ken Milroy, the chair of Colleges Scotland, said: "This is a real emergency for colleges coming after years of underinvestment."
He spoke out on the issue ahead of Thursday's debate on the Scottish budget for 2022-23 - and said it should "focus on the perilous choices some colleges will now have to make, and on the significant challenge this draft budget poses for the next academic year".
Mr Milroy said: "The total funding loss of £51.9 million means, for example, freezing recruitment, outsourcing support activities, possible changes to temporary contracts, increasing class sizes and consolidating classes.
"All of these would be a last resort, however, after years of under investment, this budget leaves no more space to manoeuvre in.
"Unless there are improvements to the budget, noticeable changes to colleges are inevitable."
He continued: "We believe this risks Scotland sliding into a skills emergency. Even in the midst of a pandemic, colleges have provided world class education and training to hundreds of thousands of students.
"But in order to continue there has to be investment based in reality. Scotland's recovery depends on creating year after year a workforce that is qualified and able - but with the proposed budget settlement, colleges will be challenged in ways not seen for many years."
Higher and Further Education Minister Jamie Hepburn said the Scottish Government recognises the importance of funding for colleges.
He pointed to the government's 2022-23 Budget which he said will provide more than £1.9 billion for Scotland's universities and colleges.
Mr Hepburn said: "Throughout the pandemic colleges have demonstrated resilience and flexibility to continue delivering the courses to produce a skilled workforce.
"They are instrumental in any economic recovery strategy - working with the small and medium sized enterprise base on upskilling and reskilling."
He added: "We know our colleges face significant financial challenges which have been exacerbated by the pandemic but we are working closely with them to mitigate the effects of the crisis."
There are currently 26 colleges operating across Scotland, which deliver 70 million hours of learning to around 239,000 learners every year, according to Colleges Scotland.