Manche Kollegen sind ein wenig neidisch auf ihren Erfolg.

Manche means roughly “some” in a general, somewhat vague way. It suggests “not all, but a noticeable number”.

Comparisons:

• manche Kollegen – some colleagues (fairly neutral, can feel a bit more general or slightly literary/formal than einige)
• einige Kollegen – some / several colleagues (often implies a limited but not tiny number)
• ein paar Kollegen – literally a few colleagues; sounds more casual and often suggests a smallish number

All three are possible here, but:

• manche and einige are closer in tone.
• ein paar can sound more like “a couple of / a few”, often slightly more informal.