彤 and 彡 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar
in older Chinese.
Character Evolution
BronzeLate Western Zhou~800 BCBronzeLate Western Zhou~800 BCBronzeLate Western Zhou~800 BCSealWarring States (Chu)475-221 BCSealShuowen~100 ADClericalQin221-206 BCClericalWestern Han202 BC-9 ADClericalJin266-420 ADRegularModern