A practical English approximation is “MAH-chuh”, with stress on the first syllable.
Say it simply
Use two syllables: mah, then chuh. English accents vary, so this approximation is a guide rather than a claim to exact native pronunciation.
What it refers to
On this site, matcha refers to a powdered green tea preparation. Botanical colour names require an ingredient check because not every “matcha” drink contains tea.